[Bug 84715] prefered applications crashes with invalidly large memory allocation

[Martin Pitt]

Public bug reported:

gnome-default-applications-properties crashes very often when using an
user-defined browser.

This sequence reproduces it for me reliably:
- click on 'user defined'
- empty command field
- click on 'user defined' again

GLib-ERROR **: gmem.c:135: failed to allocate 18446744073699015073 bytes
aborting...
Aborted

** Affects: control-center (Ubuntu)
     Importance: Undecided
         Status: Unconfirmed

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prefered applications crashes with invalidly large memory allocation
https://launchpad.net/bugs/84715