Segfault with certain AppStream data

Looks like this is causing AppCenter to segfault when trying to view the app's details page.

https://github.com/naaando/rush/blob/646c82e464b02a4e2a84dfb709c8c7a540f671ce/data/com.github.naaando.rush.appdata.xml#L30

gdb output with backtrace:

```
Starting program: /usr/bin/io.elementary.appcenter -l data/com.github.naaando.rush.appdata.xml
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7fffe6c85700 (LWP 31498)]
[New Thread 0x7fffe6484700 (LWP 31499)]
[New Thread 0x7fffe5c83700 (LWP 31500)]
[New Thread 0x7fffdf3a8700 (LWP 31505)]
[New Thread 0x7fffcf914700 (LWP 31513)]
[New Thread 0x7fffcef01700 (LWP 31516)]
[Thread 0x7fffcf914700 (LWP 31513) exited]
[Thread 0x7fffe6484700 (LWP 31499) exited]
[New Thread 0x7fffe6484700 (LWP 31525)]
[New Thread 0x7fffcf914700 (LWP 31526)]
[New Thread 0x7fffce2f6700 (LWP 31527)]
[New Thread 0x7fffcdaf5700 (LWP 31635)]
[Thread 0x7fffcdaf5700 (LWP 31635) exited]
[Thread 0x7fffe6484700 (LWP 31525) exited]
[New Thread 0x7fffe6484700 (LWP 1124)]
[New Thread 0x7fffcdaf5700 (LWP 1125)]
[New Thread 0x7fffcd2f4700 (LWP 1130)]
[Thread 0x7fffcf914700 (LWP 31526) exited]
[Thread 0x7fffce2f6700 (LWP 31527) exited]
[Thread 0x7fffe6484700 (LWP 1124) exited]
[Thread 0x7fffcdaf5700 (LWP 1125) exited]
Thread 1 "io.elementary.a" received signal SIGSEGV, Segmentation fault.
0x00007ffff6f79ce7 in gtk_stack_set_visible_child_name () from /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
(gdb) bt
#0 0x00007ffff6f79ce7 in gtk_stack_set_visible_child_name () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#1 0x00005555555a3166 in app_center_views_app_info_view_real_update_state (base=0x555557036850, first_update=1)
at /home/cassidyjames/Projects/elementary/appcenter/src/Views/AppInfoView.vala:407
#2 0x00005555555cb6cd in app_center_abstract_app_container_update_state (self=0x555557036850, first_update=1)
at /home/cassidyjames/Projects/elementary/appcenter/src/Widgets/AppContainers/AbstractAppContainer.vala:291
#3 0x00005555555cb5ee in app_center_abstract_app_container_real_set_up_package (self=0x555557036850, icon_size=128)
at /home/cassidyjames/Projects/elementary/appcenter/src/Widgets/AppContainers/AbstractAppContainer.vala:288
#4 0x00005555555cb661 in app_center_abstract_app_container_set_up_package (self=0x555557036850, icon_size=128)
at /home/cassidyjames/Projects/elementary/appcenter/src/Widgets/AppContainers/AbstractAppContainer.vala:250
#5 0x00005555555aae5b in app_center_views_app_info_view_constructor (type=93825045355152, n_construct_properties=1, construct_properties=0x555556026300) at /home/cassidyjames/Projects/elementary/appcenter/src/Views/AppInfoView.vala:385
#6 0x00007ffff78d157e in () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#7 0x00007ffff78d35c0 in g_object_new_valist () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#8 0x00007ffff78d3939 in g_object_new () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#9 0x00005555555a3028 in app_center_views_app_info_view_construct (object_type=93825045355152, package=0x555556f7aa40)
at /home/cassidyjames/Projects/elementary/appcenter/src/Views/AppInfoView.vala:43
#10 0x00005555555a3055 in app_center_views_app_info_view_new (package=0x555556f7aa40)
at /home/cassidyjames/Projects/elementary/appcenter/src/Views/AppInfoView.vala:42
#11 0x00005555555b75f5 in app_center_view_real_show_package (self=0x5555572b62e0, package=0x555556f7aa40)
at /home/cassidyjames/Projects/elementary/appcenter/src/Views/View.vala:42
#12 0x00005555555b2127 in app_center_homepage_real_show_package (base=0x5555572b62e0, package=0x555556f7aa40)
at /home/cassidyjames/Projects/elementary/appcenter/src/Views/Homepage.vala:290
#13 0x00005555555b7824 in app_center_view_show_package (self=0x5555572b62e0, package=0x555556f7aa40)
at /home/cassidyjames/Projects/elementary/appcenter/src/Views/View.vala:32
#14 0x00005555555b2d32 in _app_center_homepage___lambda144_ (self=0x5555572b62e0)
at /home/cassidyjames/Projects/elementary/appcenter/src/Views/Homepage.vala:63
#15 0x00005555555b2d6c in __app_center_homepage___lambda144__gtk_button_clicked (_sender=0x5555572ba2a0, self=0x5555572b62e0)
at /home/cassidyjames/Projects/elementary/appcenter/src/Views/Homepage.vala:60
#16 0x00007ffff78cc346 in () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#17 0x00007ffff78e79ff in g_signal_emit_valist () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#18 0x00007ffff78e812f in g_signal_emit () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#19 0x00007ffff6dcc6dd in () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#20 0x00007ffff6dcc735 in () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
---Type to continue, or q to quit---
#21 0x00007ffff78cc10d in g_closure_invoke () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#22 0x00007ffff78df12e in () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#23 0x00007ffff78e7715 in g_signal_emit_valist () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#24 0x00007ffff78e812f in g_signal_emit () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#25 0x00007ffff6dcab90 in () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#26 0x00007ffff4cecdae in ffi_call_unix64 () at /usr/lib/x86_64-linux-gnu/libffi.so.6
#27 0x00007ffff4cec71f in ffi_call () at /usr/lib/x86_64-linux-gnu/libffi.so.6
#28 0x00007ffff78ccced in g_cclosure_marshal_generic_va () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#29 0x00007ffff78cc346 in () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#30 0x00007ffff78e79ff in g_signal_emit_valist () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#31 0x00007ffff78e812f in g_signal_emit () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#32 0x00007ffff6e87476 in () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#33 0x00007ffff78cf008 in g_cclosure_marshal_VOID__BOXEDv () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#34 0x00007ffff78cc346 in () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#35 0x00007ffff78e79ff in g_signal_emit_valist () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#36 0x00007ffff78e812f in g_signal_emit () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#37 0x00007ffff6e8474e in () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#38 0x00007ffff6e85d3b in () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#39 0x00007ffff6e8899e in () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#40 0x00007ffff6e561e1 in gtk_event_controller_handle_event () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#41 0x00007ffff701585b in () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#42 0x00007ffff6ed0257 in () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#43 0x00007ffff78cc346 in () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#44 0x00007ffff78e73cd in g_signal_emit_valist () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#45 0x00007ffff78e812f in g_signal_emit () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#46 0x00007ffff7017b04 in () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#47 0x00007ffff6ecd1ce in () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#48 0x00007ffff6ecf2a8 in gtk_main_do_event () at /usr/lib/x86_64-linux-gnu/libgtk-3.so.0
#49 0x00007ffff69e0765 in () at /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#50 0x00007ffff6a10f82 in () at /usr/lib/x86_64-linux-gnu/libgdk-3.so.0
#51 0x00007ffff75f2387 in g_main_context_dispatch () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#52 0x00007ffff75f25c0 in () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#53 0x00007ffff75f264c in g_main_context_iteration () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
---Type to continue, or q to quit---
#54 0x00007ffff6488e3d in g_application_run () at /usr/lib/x86_64-linux-gnu/libgio-2.0.so.0
#55 0x0000555555569a3a in _vala_main (args=0x7fffffffdbc8, args_length1=3)
at /home/cassidyjames/Projects/elementary/appcenter/src/Application.vala:335
#56 0x0000555555569a7d in main (argc=3, argv=0x7fffffffdbc8)
at /home/cassidyjames/Projects/elementary/appcenter/src/Application.vala:333
```