OSV-2020-38 Heap-buffer-overflow in OGRFeatureFormatDateTimeBuffer

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=23417 Crash type: Heap-buffer-overflow WRITE 2 Crash state: OGRFeatureFormatDateTimeBuffer OGRFeature::GetFieldAsString OGRMemLayer::AlterFieldDefn...