Changes

rtp2trau HR to DL: validate ToC octet of RFC 5993

This validation serves two purposes:

* Payloads that just happen to be 15 bytes long but aren't valid
  RFC 5993 should be rejected;

* Super-5993 extensions of TW-TS-002 are valid only in UL, not in DL
  - catch and block them.

Change-Id: Ibbaa1e1e12254eaf75a999dd1b58e2145eff158c