Fix uninitialized variable in dshash.c.
A bugfix for commit
8c0d7bafad36434cb08ac2c78e69ae72c194ca20. 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