s4:messaging: allow imessaging_post_handler() to free the messaging context from...

s4:messaging: allow imessaging_post_handler() to free the messaging context from a handler

In usecases like using messaging_client_init() with irpc processing we may
free the imessaging_context during the messaging handler.
imessaging_post_handler() is not yet really used, but it will change in
the next commits. imessaging_post_state is a child of imessaging_context
and might be implicitly free'ed before the explicit TALLOC_FREE(state).

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>