OSV-2021-349 Heap-buffer-overflow in flatbuffers::vector_downward::fill

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=30684 Crash type: Heap-buffer-overflow WRITE 1 Crash state: flatbuffers::vectordownward::fill flatbuffers::FlatBufferBuilder::PreAlign flatbuffers::FlatBufferBuilder::ForceVectorAlignment...