Usually the values of the previous and next pointers of deleted buckets
are never used, as they're not part of the main ordered bucket chain,
but if an in-place rehashing is done, which results in the bucket being
turned into a free bucket, the stale pointers will remain, at which
point any item that is inserted into said free-bucket will have either
a stale previous pointer if the HashTable was empty on insertion, or a
stale next pointer, resulting in undefined behaviour.
This commit also includes a new HashMap test that reproduces this issue
eb02425ef9