Skip m= blocks in the SDP if the media type is unknown. This prevents
subsequent a= lines from the m= block to be applied to the previous
m= line, thus breaking otherwise functional RTP streams. See discussion in
[PATCH] RTSP-MS 7/15: parse and allow unknown m= line codes" thread on
mailinglist.

Originally committed as revision 16737 to svn://svn.ffmpeg.org/ffmpeg/trunk
