Fix num_4x4_blk scale in var-tx and cb4x4 mode

This resolves an out-of-boundary memory access issue in the encoding
process.

Change-Id: I9363f5a5a012880289e3370f66507126c609a41f

av1/encoder/context_tree.c

diff --git a/av1/encoder/context_tree.c b/av1/encoder/context_tree.c
index 561ed9c..42a803a 100644
--- a/av1/encoder/context_tree.c
+++ b/av1/encoder/context_tree.c
@@ -30,7 +30,12 @@
   const int num_blk = (num_4x4_blk < 4 ? 4 : num_4x4_blk);
   const int num_pix = num_blk * tx_size_2d[0];
   int i;
+#if CONFIG_CB4X4 && CONFIG_VAR_TX
+  ctx->num_4x4_blk = num_blk / 4;
+#else
   ctx->num_4x4_blk = num_blk;
+#endif
+
 #if CONFIG_EXT_PARTITION_TYPES
   ctx->partition = partition;
 #endif