Use the control URI from the SDP (if present) rather than the input filename,

if present. This fixes playback of a number of MS-RTSP streams, mostly these
for which playback contains a session key in the URI. Fixes issue 1697.
Patch by Alan Steremberg <$firstname dot $lastname () gmail com>.

Originally committed as revision 21381 to svn://svn.ffmpeg.org/ffmpeg/trunk