Initialize skip_txfm_sb flag in motion_mode_rd()

BUG=aomedia:1172

Change-Id: I7fcc3a7786acd272f46d8a675e3bd5d80a52f5bc
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 45d7ba1..a687ed9 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -7401,7 +7401,7 @@
     int64_t tmp_dist;
     int tmp_rate2 =
         motion_mode != SIMPLE_TRANSLATION ? rate2_bmc_nocoeff : rate2_nocoeff;
-
+    *skip_txfm_sb = 0;
     *mbmi = base_mbmi;
     mbmi->motion_mode = motion_mode;
     if (mbmi->motion_mode == OBMC_CAUSAL) {