OSV-2021-510 Heap-use-after-free in grk_object_unref

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=31874 Crash type: Heap-use-after-free READ 8 Crash state: grkobjectunref grk::GrkObjectImpl::release grk::GrkObject::unref...