cvdsub: use VLC_TS_INVALID (refs #3135)

modules/codec/cvdsub.c

@@ -230,7 +230,7 @@ static block_t *Reassemble( decoder_t *p_dec, block_t *p_block )
      * to detect the first packet in a subtitle.  The first packet
      * seems to have a valid PTS while later packets for the same
      * image don't. */
-    if( p_sys->i_state == SUBTITLE_BLOCK_EMPTY && p_block->i_pts == 0 )
+    if( p_sys->i_state == SUBTITLE_BLOCK_EMPTY && p_block->i_pts <= VLC_TS_INVALID )
     {
         msg_Warn( p_dec, "first packet expected but no PTS present");
         return NULL;