Random crashes when browsing a directory with incompatible files.

This is difficult to reproduce.

With 0.12.3 (and 0.12.2), when navigating from a compatible to an incompatible file, Ristretto either:

• remove that file unreliably, or
• reach a segmentation fault.

In the first case:

(ristretto:528107): GLib-GObject-CRITICAL **: 20:55:34.542: g_object_unref: assertion 'G_IS_OBJECT (object)' failed

** (ristretto:528107): WARNING **: 20:55:34.825: (thumbnailer.c:251):rstto_thumbnailer_get_timer_flavor: code should not be reached

In the second case on gdb (may not be helpful):

Thread 1 "ristretto" received signal SIGSEGV, Segmentation fault.
0x00007ffff7263c8d in g_type_check_instance_is_fundamentally_a ()
   from /lib/x86_64-linux-gnu/libgobject-2.0.so.0

Attached is an archive of the test directory (actually part of the default Xfwm4 theme).

png+xpm.zip