thumbnailer: Fix a potential NULL pointer dereference
thumbnailer: Fix a potential NULL pointer dereference

In debug code on an error path: if the loader is not loaded due to
mime_type being NULL, no error is set; so dereferencing it is a bad
idea.

Coverity CID 1388530

https://bugzilla.gnome.org/show_bug.cgi?id=776026

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf