smbd: Always clean up the child's msg_ctx

smbd: Always clean up the child's msg_ctx

This is a bit lazy programming, we could and possibly should do this in
exit_server() in the child. But this way we make sure the cleanup works. If it
only was executed for unclean exits, we might not detect failure of this code
in the parent.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>