printers: Move ownership of GHashTable to the recipient

Once we call callback passed to the get_ipp_attributes_async()
we should not try to free produced data but keep this to the
caller.

Fixes #2843

panels/printers/pp-utils.c

@@ -1213,6 +1213,7 @@ get_ipp_attributes_idle_cb (gpointer user_data)
   GIAData *data = (GIAData *) user_data;
 
   data->callback (data->result, data->user_data);
+  data->result = NULL;
 
   return FALSE;
 }