thumbnailer: Remove duplicate function declarations

This shuts up a gcc warning.

https://bugzilla.gnome.org/show_bug.cgi?id=709819
thumbnailer: Fix a signed/unsigned comparison

This shuts up a gcc warning but shouldn’t result in functional changes.

https://bugzilla.gnome.org/show_bug.cgi?id=709819
thumbnailer: Bail if no pixbuf loader could be created

This prevents gdk_pixbuf_loader_close() from emitting a critical
error due to being called with a NULL loader. The loader can be NULL
if there was an error in the first read from the input stream.

https://bugzilla.gnome.org/show_bug.cgi?id=709819