OSV-2020-1228 Heap-use-after-free in arrow::internal::CountSetBits

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=20120 Crash type: Heap-use-after-free READ 8 Crash state: arrow::internal::CountSetBits arrow::Array::nullcount arrow::internal::ValidateArray...