[runtime.c] Application crashes inside the AppImage, works when extracted

https://github.com/probonopd/nasc/releases/tag/continuous on `neon-useredition-20160609-0826-amd64.iso`:

```
me@host:~$ /home/me/Downloads/NaSC-1a6a313-x86_64.AppImage
GST_PLUGIN_SCANNER=/tmp/.mount_NaSC-10hb3Nw/usr/lib/x86_64-linux-gnu/gstreamer1.0/gstreamer-1.0/gst-plugin-scanner
GST_PLUGIN_SYSTEM_PATH=/tmp/.mount_NaSC-10hb3Nw/lib/x86_64-linux-gnu/gstreamer-1.0
GST_PLUGIN_PATH=/tmp/.mount_NaSC-10hb3Nw/lib/x86_64-linux-gnu/gstreamer-1.0
Run experimental self-contained bundle
[INFO 23:15:48.085472] Application.vala:156: NaSC version: 0.7.4
[INFO 23:15:48.085593] Application.vala:158: Kernel version: 4.4.0-22-generic

(com.github.parnold-x.nasc:8978): Gtk-WARNING **: 23:15:48.090: Locale not supported by C library.
Using the fallback 'C' locale.
Fontconfig warning: "/etc/fonts/conf.avail/53-monospace-lcd-filter.conf", line 10: Having multiple values in isn't supported and may not work as expected

(...)

(com.github.parnold-x.nasc:8978): Gtk-WARNING **: 23:15:48.297: Theme directory devices/48 of theme breeze has no size field

Segmentation fault (core dumped)
```

Now the strange part:

The same application, when extracted with `--appimage-extract`, runs.

__Why?__