Fix an issue where media would not flow for situations where the legacy STUN code is in use.

The STUN packets should *not* be blocked by strict RTP.

(closes issue ASTERISK-20102)
Reported by: Malcolm Davenport


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369817 65c4cc65-6c06-0410-ace0-fbb531ad65f3