app: Unref GtkFileFilter only when a file loader has extensions set

gimp_file_proc_view_process_procedure() can return NULL in case a file
loading procedure does not have any extensions set.

app/widgets/gimpfileprocview.c

@@ -164,7 +164,8 @@ gimp_file_proc_view_new (Gimp        *gimp,
                                   COLUMN_HELP_ID,    help_id,
                                   COLUMN_FILTER,     filter,
                                   -1);
-              g_object_unref (filter);
+              if (filter != NULL)
+                g_object_unref (filter);
             }
 
           for (list2 = proc->extensions_list;