librpc/rpc: let dcerpc_read_ncacn_packet_next_vector() handle fragments without any...

librpc/rpc: let dcerpc_read_ncacn_packet_next_vector() handle fragments without any payload

DCERPC_PKT_CO_CANCEL and DCERPC_PKT_ORPHANED don't have any payload by
default. In order to receive them via dcerpc_read_ncacn_packet_send/recv
we need to allow fragments with frag_len == DCERPC_NCACN_PAYLOAD_OFFSET.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15446

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>