CVE-2015-5370: s3:rpc_client: remove useless frag_length check in rpc_api_pipe_got_pdu()

dcerpc_pull_ncacn_packet() already verifies this.

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>
