Fix uninitialized entropy contexts

When enable_optimize_b is false in av1_encode_intra_block_plane the
entropy contexts were never initialized.

No changes on metrics for objective-1-fast when no experiment is
enabled.

Change-Id: Ic68913f6400d2becbaec3cc14214a0257530ed0b

av1/encoder/encodemb.c

diff --git a/av1/encoder/encodemb.c b/av1/encoder/encodemb.c
index e2c6144..3a52303 100644
--- a/av1/encoder/encodemb.c
+++ b/av1/encoder/encodemb.c
@@ -1092,8 +1092,8 @@
                                   BLOCK_SIZE bsize, int plane,
                                   int enable_optimize_b) {
   const MACROBLOCKD *const xd = &x->e_mbd;
-  ENTROPY_CONTEXT ta[2 * MAX_MIB_SIZE];
-  ENTROPY_CONTEXT tl[2 * MAX_MIB_SIZE];
+  ENTROPY_CONTEXT ta[2 * MAX_MIB_SIZE] = { 0 };
+  ENTROPY_CONTEXT tl[2 * MAX_MIB_SIZE] = { 0 };
 
   struct encode_b_args arg = {
     cm, x, NULL, &xd->mi[0]->mbmi.skip, ta, tl, enable_optimize_b