Do not call g_object_unref when imageType comes out as UNKNOWN.

Avoids "(sharp:23220): GLib-GObject-CRITICAL **: g_object_unref: assertion 'G_IS_OBJECT (object)' failed" dumped to the console when an unsupported or invalid image is loaded.
Merge pull request #107 from papandreou/doNotUnrefUnparsableImages

Do not call g_object_unref when imageType comes out as UNKNOWN.