[gsignal] fix up a crasher in previous commit

The previous commit introduced a new variable in the Handler struct but
didn't initialise it.  This was causing some tests to crash.

https://bugzilla.gnome.org/show_bug.cgi?id=118536

gobject/gsignal.c

@@ -587,6 +587,7 @@ handler_new (gboolean after)
   handler->block_count = 0;
   handler->after = after != FALSE;
   handler->closure = NULL;
+  handler->has_invalid_closure_notify = 0;
   
   return handler;
 }