avcodec/sonic: Check channels before deallocating

Fixes: heap-buffer-overflow
Fixes: 25744/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SONIC_fuzzer-5172961169113088

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit f249981976)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/sonic.c

@@ -980,9 +980,7 @@ static av_cold int sonic_decode_close(AVCodecContext *avctx)
     av_freep(&s->int_samples);
     av_freep(&s->tap_quant);
     av_freep(&s->predictor_k);
-
-    for (i = 0; i < s->channels; i++)
-    {
+    for (i = 0; i < MAX_CHANNELS; i++) {
         av_freep(&s->predictor_state[i]);
         av_freep(&s->coded_samples[i]);
     }