Clear a static analysis warning in encode_sb()

BUG=aomedia:2312

Change-Id: I8d309ad8953b9d07a8879a9565f786d292a44dc1
diff --git a/av1/encoder/encodeframe.c b/av1/encoder/encodeframe.c
index aaa1463..263a2ce 100644
--- a/av1/encoder/encodeframe.c
+++ b/av1/encoder/encodeframe.c
@@ -1589,6 +1589,7 @@
                       TileDataEnc *tile_data, TOKENEXTRA **tp, int mi_row,
                       int mi_col, RUN_TYPE dry_run, BLOCK_SIZE bsize,
                       PC_TREE *pc_tree, int *rate) {
+  assert(bsize < BLOCK_SIZES_ALL);
   const AV1_COMMON *const cm = &cpi->common;
   MACROBLOCK *const x = &td->mb;
   MACROBLOCKD *const xd = &x->e_mbd;