avcodec/siren: Increase noise category 5 and 6

The entry read is not used in subsequent computation, thus its
value is not important.

Fixes: out of array read
Fixes: 28578/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SIREN_fuzzer-6332019122503680

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

libavcodec/siren.c

@@ -342,12 +342,12 @@ static const float mlt_quant[7][14] = {
     { 0.0f, 1.964f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f }
 };
 
-static const float noise_category5[20] = {
+static const float noise_category5[21] = {
     0.70711f, 0.6179f, 0.5005f, 0.3220f, 0.17678f, 0.17678f, 0.17678f, 0.17678f, 0.17678f, 0.17678f, 0.17678f,
     0.17678f, 0.17678f, 0.17678f, 0.17678f, 0.17678f, 0.17678f, 0.17678f, 0.17678f, 0.17678f
 };
 
-static const float noise_category6[20] = {
+static const float noise_category6[21] = {
     0.70711f, 0.5686f, 0.3563f, 0.25f, 0.25f, 0.25f, 0.25f, 0.25f, 0.25f, 0.25f, 0.25f, 0.25f,
     0.25f, 0.25f, 0.25f, 0.25f, 0.25f, 0.25f, 0.25f, 0.25f
 };