subsusf: use VLC_TS_INVALID (refs #3135)

modules/codec/subtitles/subsusf.c

@@ -182,7 +182,7 @@ static subpicture_t *ParseText( decoder_t *p_dec, block_t *p_block )
     char *psz_subtitle = NULL;
 
     /* We cannot display a subpicture with no date */
-    if( p_block->i_pts == 0 )
+    if( p_block->i_pts <= VLC_TS_INVALID )
     {
         msg_Warn( p_dec, "subtitle without a date" );
         return NULL;