avcodec/qdm2: Check fft_coefs_index

Fixes: out of array access
Fixes: 20660/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_QDM2_fuzzer-5658290216501248

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>
(cherry picked from commit 9fc73bf022)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/qdm2.c

@@ -1334,6 +1334,9 @@ static void qdm2_fft_decode_tones(QDM2Context *q, int duration,
         if (q->frequency_range > (local_int_14 + 1)) {
             int sub_packet = (local_int_20 + local_int_28);
 
+            if (q->fft_coefs_index + stereo >= FF_ARRAY_ELEMS(q->fft_coefs))
+                return;
+
             qdm2_fft_init_coefficient(q, sub_packet, offset, duration,
                                       channel, exp, phase);
             if (stereo)