messaging: Avoid crashes

messaging: Avoid crashes

With the latest patches we defer messages to a messaging_context's default
context, the one that it was created with. This is another incarnation of
085542fc93b3 (I believe): messaging contexts can outlive their tevent contexts.

In this case, the tevent_schedule_immediate(msg_ctx->event_ctx) has nothing to
schedule on and will crash. This patch uses the fact that tevent_fd's can
outlive their event_contexts. When the tevent_context dies, all tevent_fd's
will get their flags set to 0. The tevent_handles in messages_dgm_ref always
have TEVENT_FD_READ set, so a 0 flags field indicates the tevent_context has
died.

Signed-off-by: Volker Lendecke <vl@samba.org>
Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Oct  5 03:51:43 CEST 2016 on sn-devel-144