CVE-2015-5370: s4:librpc/rpc: protect dcerpc_request_recv_data() against too large payloads

We should only allow a combined payload of a response of at max 4 MBytes.

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>
