A bugfix for commit 8c0d7baf.  The code
would have crashed if hashtable->size_log2 ever had the same value as
hashtable->control->size_log2 by coincidence.

Per Valgrind.

Author: Thomas Munro
Reported-By: Tomas Vondra
Discussion: https://postgr.es/m/e72fb33c-4f31-f276-e972-263d9b59554d%402ndquadrant.com