Fix uninitialized variable in dshash.c.

A bugfix for commit 8c0d7bafad.  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

src/backend/lib/dshash.c

@@ -249,6 +249,7 @@ dshash_create(dsa_area *area, const dshash_parameters *params, void *arg)
 	}
 	hash_table->buckets = dsa_get_address(area,
 										  hash_table->control->buckets);
+	hash_table->size_log2 = hash_table->control->size_log2;
 
 	return hash_table;
 }
@@ -280,6 +281,14 @@ dshash_attach(dsa_area *area, const dshash_parameters *params,
 	hash_table->find_exclusively_locked = false;
 	Assert(hash_table->control->magic == DSHASH_MAGIC);
 
+	/*
+	 * These will later be set to the correct values by
+	 * ensure_valid_bucket_pointers(), at which time we'll be holding a
+	 * partition lock for interlocking against concurrent resizing.
+	 */
+	hash_table->buckets = NULL;
+	hash_table->size_log2 = 0;
+
 	return hash_table;
 }