RTSP: Shrink SDP fmtp parsing buffer size

Since the parsing of Vorbis/Theora fmtp headers is handled by the
parse_sdp_a_line function pointer now, the buffer in sdp_parse_fmtp
doesn't need to be this large any longer.

Patch by Josh Allmann, joshua dot allmann at gmail

Originally committed as revision 23599 to svn://svn.ffmpeg.org/ffmpeg/trunk