PS: fix duration computation on some files

Ref #9412

modules/demux/ps.c

@@ -255,7 +255,7 @@ static void FindLength( demux_t *p_demux )
 
         /* Check end */
         i_size = stream_Size( p_demux->s );
-        i_end = VLC_CLIP( i_size, 0, 200000 );
+        i_end = VLC_CLIP( i_size, 0, 20000 );
         stream_Seek( p_demux->s, i_size - i_end );
 
         i = 0;