avcodec/adpcm: Clip step index for ADPCM_IMA_APM

Fixes: out of array access
Fixes: 20828/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ADPCM_IMA_APM_fuzzer-5712770106654720

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

libavcodec/adpcm.c

@@ -154,9 +154,9 @@ static av_cold int adpcm_decode_init(AVCodecContext * avctx)
     case AV_CODEC_ID_ADPCM_IMA_APM:
         if (avctx->extradata && avctx->extradata_size >= 16) {
             c->status[0].predictor  = AV_RL32(avctx->extradata +  0);
-            c->status[0].step_index = AV_RL32(avctx->extradata +  4);
+            c->status[0].step_index = av_clip(AV_RL32(avctx->extradata +  4), 0, 88);
             c->status[1].predictor  = AV_RL32(avctx->extradata +  8);
-            c->status[1].step_index = AV_RL32(avctx->extradata + 12);
+            c->status[1].step_index = av_clip(AV_RL32(avctx->extradata + 12), 0, 88);
         }
         break;
     case AV_CODEC_ID_ADPCM_IMA_WS: