avcodec/atrac9dec: Check that the reused block has succeeded initilization

Fixes: global-buffer-overflow
Fixes: 15247/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ATRAC9_fuzzer-5671602181636096

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

libavcodec/atrac9dec.c

@@ -71,6 +71,8 @@ typedef struct ATRAC9BlockData {
     int cpe_base_channel;
     int is_signs[30];
 
+    int reuseable;
+
 } ATRAC9BlockData;
 
 typedef struct ATRAC9Context {
@@ -668,6 +670,7 @@ static int atrac9_decode_block(ATRAC9Context *s, GetBitContext *gb,
     if (!reuse_params) {
         int stereo_band, ext_band;
         const int min_band_count = s->samplerate_idx > 7 ? 1 : 3;
+        b->reuseable = 0;
         b->band_count = get_bits(gb, 4) + min_band_count;
         b->q_unit_cnt = at9_tab_band_q_unit_map[b->band_count];
 
@@ -699,6 +702,11 @@ static int atrac9_decode_block(ATRAC9Context *s, GetBitContext *gb,
             }
             b->band_ext_q_unit = at9_tab_band_q_unit_map[ext_band];
         }
+        b->reuseable = 1;
+    }
+    if (!b->reuseable) {
+        av_log(s->avctx, AV_LOG_ERROR, "invalid block reused!\n");
+        return AVERROR_INVALIDDATA;
     }
 
     /* Calculate bit alloc gradient */