avcodec/bitstream: Check for integer code truncation in build_table()

Fixes: out of array read
Fixes: 14563/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AGM_fuzzer-5646451545210880

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/bitstream.c

@@ -226,6 +226,10 @@ static int build_table(VLC *vlc, int table_nb_bits, int nb_codes,
             /* note: realloc has been done, so reload tables */
             table = (volatile VLC_TYPE (*)[2])&vlc->table[table_index];
             table[j][0] = index; //code
+            if (table[j][0] != index) {
+                avpriv_request_sample(NULL, "strange codes");
+                return AVERROR_PATCHWELCOME;
+            }
             i = k-1;
         }
     }