CVE-2015-5370: s4:librpc/rpc: call dcerpc_connection_dead() on protocol errors

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11344

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
