OSV-2020-1058 Heap-buffer-overflow in arrow::UnionType::UnionType

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=20116 Crash type: Heap-buffer-overflow WRITE 4 Crash state: arrow::UnionType::UnionType arrow::union arrow::ipc::internal::ConcreteTypeFromFlatbuffer...