Re-fix sending unnegotiated payloads during a P2P RTP bridge.

The previous fix still would look in the static_RTP_PT table, which
is inappropriate since we specifically want to find a codec that has
been negotiated.

(closes issue ASTERISK-20296)
reported by NITESH BANSAL
Patches:
	codec_negotiation.patch Uploaded by NITESH BANSAL (License #6418)
........

Merged revisions 372311 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@372319 65c4cc65-6c06-0410-ace0-fbb531ad65f3