rpc: Prevent "malformed" for RPC 'NULL proc' packets.

Bug: 10646
Change-Id: Ifb96768969a43c59367b73e7bb940ceeb02cf6b0
Reviewed-on: https://code.wireshark.org/review/5700
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>