Force skipping transform coding in skip_mode

BUG=aomedia:1530

Change-Id: I6d1fd61951a64492dec7e7b86c7460efa93eb6f7
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index da86d48..993e456 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -1015,12 +1015,8 @@
 
   write_skip_mode(cm, xd, segment_id, mi, w);
 
-  if (mbmi->skip_mode) {
-    skip = mbmi->skip;
-    assert(skip);
-  } else {
-    skip = write_skip(cm, xd, segment_id, mi, w);
-  }
+  assert(IMPLIES(mbmi->skip_mode, mbmi->skip));
+  skip = mbmi->skip_mode ? 1 : write_skip(cm, xd, segment_id, mi, w);
 
 #if CONFIG_SPATIAL_SEGMENTATION
   write_inter_segment_id(cpi, w, seg, segp, mi_row, mi_col, skip, 0);