Use correct segment

Segmment based lossless flag is used in select transform size, this
commit fixes a bug where wrong segment_id is used in such selection.

BUG=aomedia:350

Change-Id: Ibc981c779739849bac00447155180abbd319eb28

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 7b84fd2..835f56b 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -2232,7 +2232,7 @@
 
   assert(bs == xd->mi[0]->mbmi.sb_type);
 
-  if (xd->lossless[0]) {
+  if (xd->lossless[xd->mi[0]->mbmi.segment_id]) {
     choose_smallest_tx_size(cpi, x, rd_stats, ref_best_rd, bs);
   } else if (cpi->sf.tx_size_search_method == USE_LARGESTALL) {
     choose_largest_tx_size(cpi, x, rd_stats, ref_best_rd, bs);