OSV-2021-1409 Heap-buffer-overflow in unpack_16bit_4chan_planar

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=39538 Crash type: Heap-buffer-overflow WRITE 2 Crash state: unpack16bit4chanplanar exrdecodingrun Imf31::checkCoreFile...