live555: fix seeking

According to RFC 2326 a PAUSE command should be issued when requesting an immediate seek to a new PLAY position.
(cherry picked from commit 6c29937d)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/live555.cpp

@@ -1284,7 +1284,8 @@ static int Control( demux_t *p_demux, int i_query, va_list args )
                     time = f * (double)p_sys->i_npt_length;   /* in second */
                 }
 
-                if( !p_sys->rtsp->playMediaSession( *p_sys->ms, time, -1, 1 ) )
+                if( !p_sys->rtsp->pauseMediaSession( *p_sys->ms ) ||
+                    !p_sys->rtsp->playMediaSession( *p_sys->ms, time, -1, 1 ) )
                 {
                     msg_Err( p_demux, "PLAY failed %s",
                         p_sys->env->getResultMsg() );