edbus: Avoid call _on_signal_handler_free over freed object or proxy

edbus: Avoid call _on_signal_handler_free over freed object or proxy

If a signal handler callback remove the last reference of his parent, his parent
will unref all your childrens including signal handler, but signal handler could
not be freed until your callback was finish so a event to delete it will be
created and when this event happen the signal handler parent was already freed
and we could not call _on_signal_handler_free().

Patch by: José Roberto de Souza  <zehortigoza@profusion.mobi>

SVN revision: 78198