Disallow 4:4:0 subsampling

BUG=aomedia:1233

Change-Id: Ib740ee063b87003cf8c6cbd8f2ae4b917892e41c

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 48cc8fa..a939e60 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -3303,7 +3303,12 @@
       if (cm->bit_depth == AOM_BITS_12) {
         // 420, 444 or 422
         aom_wb_write_bit(wb, cm->subsampling_x);
-        aom_wb_write_bit(wb, cm->subsampling_y);
+        if (cm->subsampling_x == 0) {
+          assert(cm->subsampling_y == 0 &&
+                 "4:4:0 subsampling not allowed in AV1");
+        } else {
+          aom_wb_write_bit(wb, cm->subsampling_y);
+        }
       } else {
         // 422 only
         assert(cm->subsampling_x == 1 && cm->subsampling_y == 0);