Crash when trying to open a long conversation (+49 messages)

Hello dear contributors of the Mail app for Elementary,
thank you very much for the great common you're nurturing. I am using it everyday and it's perfect.

I have been encountering a bug recently on mail with a conversation which has many replies in it. This conversation gathers at the moment 52 messages given the counter displayed but this started when it was around 49 I believe.

When I click on the conversation on the left panel, it looks like it's opening the conversation on the right panel but a second after the app crash.

Here's what I get in syslog :
```

Jul 17 09:57:25 imacrea pantheon-mail[31482]: The program 'pantheon-mail' received an X Window System error.#012This probably reflects a bug in the program.#012The error was 'BadAlloc (insufficient resources for operation)'.#012 (Details: serial 14219 error_code 11 request_code 53 (core protocol) minor_code 0)#012 (Note to programmers: normally, X errors are reported asynchronously;#012 that is, you will receive the error a while after causing it.#012 To debug your program, run it with the GDK_SYNCHRONIZE environment#012 variable to change this behavior. You can then get a meaningful#012 backtrace from your debugger if you break on the gdk_x_error() function.)
Jul 17 09:57:25 imacrea kernel: [62972.433497] traps: pantheon-mail[31482] trap int3 ip:7f3445ef9ea1 sp:7ffca255b2c0 error:0 in libglib-2.0.so.0.5600.4[7f3445ea8000+114000]
```