lpcm: use VLC_TS_INVALID (refs #3135)

modules/codec/lpcm.c

@@ -268,7 +268,7 @@ static void *DecodeFrame( decoder_t *p_dec, block_t **pp_block )
     *pp_block = NULL; /* So the packet doesn't get re-sent */
 
     /* Date management */
-    if( p_block->i_pts > 0 &&
+    if( p_block->i_pts > VLC_TS_INVALID &&
         p_block->i_pts != date_Get( &p_sys->end_date ) )
     {
         date_Set( &p_sys->end_date, p_block->i_pts );