Fix more warnings from static analysis

With this cange the number of shallow analysis warnings is 0 and the number of
deep analysis warnings is 18

Change-Id: Iba04f7d35ad6db60c97162a2416b997bf2713ce9
diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index f4825dd..7849a5c 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -1734,6 +1734,7 @@
 static void decode_partition(AV1Decoder *const pbi, ThreadData *const td,
                              int mi_row, int mi_col, aom_reader *reader,
                              BLOCK_SIZE bsize, int parse_decode_flag) {
+  assert(bsize < BLOCK_SIZES_ALL);
   AV1_COMMON *const cm = &pbi->common;
   MACROBLOCKD *const xd = &td->xd;
   const int bw = mi_size_wide[bsize];