gdkpixbufdec: free query after use

In gst_gdk_pixbuf_dec_setup_pool(), query is being allocated using
gst_query_new_allocation(), but the same is not unreferenced
hence calling gst_query_unref() after usage of query.

https://bugzilla.gnome.org/show_bug.cgi?id=735950

ext/gdk_pixbuf/gstgdkpixbufdec.c

@@ -246,6 +246,8 @@ gst_gdk_pixbuf_dec_setup_pool (GstGdkPixbufDec * filter, GstVideoInfo * info)
     min = max = 0;
   }
 
+  gst_query_unref (query);
+
   if (pool == NULL) {
     /* we did not get a pool, make one ourselves then */
     pool = gst_buffer_pool_new ();