gcontenttype: Fix a potential g_object_unref(NULL) call

This can happen if the g_file_query_info() call fails, returning NULL.

Found with scan-build.

https://bugzilla.gnome.org/show_bug.cgi?id=113075

gio/gcontenttype.c

@@ -1325,7 +1325,7 @@ matchlet_match (TreeMatchlet *matchlet,
             result = FALSE;
         }
 
-      g_object_unref (info);
+      g_clear_object (&info);
       g_object_unref (file);
     }
   while (!result);