ps demux: reset current pts when seeking

This prevented the play time from being updated correctly in the
interface when seeking backwards
(cherry picked from commit 2ec9749b)
Signed-off-by: Pierre Ynard <linkfanel@yahoo.fr>

modules/demux/ps.c

@@ -514,6 +514,7 @@ static int Control( demux_t *p_demux, int i_query, va_list args )
                 if( !i_now )
                     return i64 ? VLC_EGENERIC : VLC_SUCCESS;
 
+                p_sys->i_current_pts = 0;
                 i_pos *= (float)i64 / (float)i_now;
                 stream_Seek( p_demux->s, i_pos );
                 return VLC_SUCCESS;