CVE-2015-5370: s3:librpc/rpc: don't call dcerpc_pull_auth_trailer() if auth_length is 0

All other paranoia checks are done within dcerpc_pull_auth_trailer()
now.

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>
