Fix Coverity defect CID 564327 in motion_mode_rd

Remove the redundant conditional check when updating
rd_stats_y->skip_txfm.

Fixes Coverity defect CID 564327 (Dereference after null check).

Change-Id: I6edb3e1bf0edfd35b0e2adc292cb63511b682ac9

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 053edda..68e95e5 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -1847,7 +1847,7 @@
       }
       mbmi->skip_txfm = 0;
       rd_stats->skip_txfm = 0;
-      if (rd_stats_y) rd_stats_y->skip_txfm = 0;
+      rd_stats_y->skip_txfm = 0;
       increase_motion_mode_rdstats(cpi, mbmi, rd_stats, NULL, NULL);
 
     } else {