Fix enc/dec mismatch due to ext-partition-type in cb4x4 mode

This commit fixes an enc/dec mismatch issue in ext-partition-type
in the cb4x4 mode.

BUG=aomedia:137

Change-Id: I19f538a967a6059a40b1668eed076bc315b46149

av1/common/onyxc_int.h

diff --git a/av1/common/onyxc_int.h b/av1/common/onyxc_int.h
index ca93bef..d0517c1 100644
--- a/av1/common/onyxc_int.h
+++ b/av1/common/onyxc_int.h
@@ -673,7 +673,7 @@
                                                 BLOCK_SIZE bsize,
                                                 PARTITION_TYPE partition) {
   if (bsize >= BLOCK_8X8) {
-    const int bsl = b_width_log2_lookup[bsize], hbs = (1 << bsl) / 4;
+    const int hbs = mi_size_wide[bsize] / 2;
     BLOCK_SIZE bsize2 = get_subsize(bsize, PARTITION_SPLIT);
     switch (partition) {
       case PARTITION_SPLIT: