Fix static analyzer warnings

With clang v7 we are getting inflow of static analyzer warnings. With
this fix the number of shallow warnigns is 0 and number of deep
warnings is 37

BUG=aomedia:2312

Change-Id: I64846cb078d48f115ee490d0a9c8e1abfa2faae7

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 2d58364..d052ac6 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -1373,6 +1373,7 @@
   MACROBLOCKD *const xd = &x->e_mbd;
   const struct macroblockd_plane *const pd = &xd->plane[plane];
   const BLOCK_SIZE bsize = mbmi->sb_type;
+  assert(bsize < BLOCK_SIZES_ALL);
   const BLOCK_SIZE bsizec =
       scale_chroma_bsize(bsize, pd->subsampling_x, pd->subsampling_y);