Free AVStream->info in chained muxers

This fixes memory leaks in the RTSP muxer and RTP hinting in the
mov muxer present since SVN rev 25418.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
(cherry picked from commit ce41c51b0c71c87f623914ba0786aef325d818fe)
