Clarify log WARNING message when port-zero SDP 'm' lines received.

Clarify log WARNING message when port-zero SDP 'm' lines received.

Previously, if an m-line in an SDP offer or answer had a port number of zero,
that line was skipped, and resulted in an 'Unsupported SDP media type...'
warning message. This was misleading, as the media type was not unsupported,
but was ignored because the m-line indicated that the media stream had been
rejected (in an answer) or was not going to be used (in an offer).

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@353260 65c4cc65-6c06-0410-ace0-fbb531ad65f3