input/es_out : use VLC_TS_INVALID (refs #3135)

src/input/es_out.c

@@ -1903,7 +1903,7 @@ static int EsOutSend( es_out_t *out, es_out_id_t *es, block_t *p_block )
     if( p_sys->i_preroll_end >= 0 )
     {
         int64_t i_date = p_block->i_pts;
-        if( i_date <= 0 )
+        if( p_block->i_pts <= VLC_TS_INVALID )
             i_date = p_block->i_dts;
 
         if( i_date < p_sys->i_preroll_end )