OSV-2021-32 Heap-use-after-free in igraph_vector_ptr_size

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=29408 Crash type: Heap-use-after-free READ 8 Crash state: igraphvectorptrsize igraphgmltreedestroy igraphigmlparsedatadestroy...