MEMORY engine needs the record length to be at least sizeof(void*),
because it stores a pointer there (linking deleted records into a list).
So when the reclength is less than sizeof(void*), it's set to sizeof(void*).
That is done inside heap_create(), and the upper layer doesn't know
that the engine writes beyond share->reclength.

While it's usually safe (in-memory record size is rounded up to
sizeof(double), so even if share->reclength is too small,
share->rec_buff_len is not), it could cause problems in the code that
copies records and expects them to fix in share->reclength,
e.g. in partitioning.