png: use VLC_TS_INVALID (refs #3135)

modules/codec/png.c

@@ -227,7 +227,7 @@ static picture_t *DecodeBlock( decoder_t *p_dec, block_t **pp_block )
     png_destroy_read_struct( &p_png, &p_info, &p_end_info );
     free( p_row_pointers );
 
-    p_pic->date = p_block->i_pts > 0 ? p_block->i_pts : p_block->i_dts;
+    p_pic->date = p_block->i_pts > VLC_TS_INVALID ? p_block->i_pts : p_block->i_dts;
 
     block_Release( p_block ); *pp_block = NULL;
     return p_pic;