avcodec/siren: Check index for catergory5

Fixes: out of array access
Fixes: 38603/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MSNSIREN_fuzzer-5741847809490944.fuzz

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

libavcodec/siren.c

@@ -648,6 +648,10 @@ static int decode_vector(SirenContext *s, int number_of_regions,
                 }
                 coefs_ptr++;
             }
+            if (i >= FF_ARRAY_ELEMS(noise_category5)) {
+                error = 1;
+                break;
+            }
 
             noise = decoder_standard_deviation[region] * noise_category5[i];
         } else