OSV-2020-981 Heap-buffer-overflow in surround51_to_stereo_downmix_mtx

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=20163 Crash type: Heap-buffer-overflow WRITE 4 Crash state: surround51tostereodownmixmtx crasfmtconvcreate configformatconverter...