Prevent unnecessary warnings when getting rtpsource or rtpdest.

Prevent unnecessary warnings when getting rtpsource or rtpdest.

If a recognized media type was present, but the media type was not
enabled for the channel, then a warning would be emitted. For instance,
attempting to get CHANNEL(rtpsource,video) on a call with no video would
cause a warning message to appear.

With this change, the warning will only appear if the stream argument
is not recognized as being a media type that can be specified.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@261313 65c4cc65-6c06-0410-ace0-fbb531ad65f3