Call check_back_and_send_rr() function only in case of RTP as a transport.
Don't call it for RDT, since it is unneeded and it doesn't provide a
RTPDemuxContext, leading to some memory errors. See "[PATCH] fix small
memory error in rtsp.c" thread on ML.

Originally committed as revision 15828 to svn://svn.ffmpeg.org/ffmpeg/trunk
