Fix uninitialized slot array access during the upgrade.

Commit 29d0a77fa introduced fetching slot information from the old cluster
but didn't initialize the required array in all the code paths. So when
trying to access the array in verbose mode for the new cluster, it leads
to an uninitialized memory access.

Author: Vignesh C
Discussion: http://postgr.es/m/CALDaNm1tntGP5=CtMz=v+k3_PGv7kE9t6iWSgX-QiurAaFkhZw@mail.gmail.com

src/bin/pg_upgrade/info.c

@@ -408,7 +408,7 @@ get_db_infos(ClusterInfo *cluster)
 	i_spclocation = PQfnumber(res, "spclocation");
 
 	ntups = PQntuples(res);
-	dbinfos = (DbInfo *) pg_malloc(sizeof(DbInfo) * ntups);
+	dbinfos = (DbInfo *) pg_malloc0(sizeof(DbInfo) * ntups);
 
 	for (tupnum = 0; tupnum < ntups; tupnum++)
 	{
@@ -636,15 +636,11 @@ get_old_cluster_logical_slot_infos(DbInfo *dbinfo, bool live_check)
 	PGconn	   *conn;
 	PGresult   *res;
 	LogicalSlotInfo *slotinfos = NULL;
-	int			num_slots = 0;
+	int			num_slots;
 
 	/* Logical slots can be migrated since PG17. */
 	if (GET_MAJOR_VERSION(old_cluster.major_version) <= 1600)
-	{
-		dbinfo->slot_arr.slots = slotinfos;
-		dbinfo->slot_arr.nslots = num_slots;
 		return;
-	}
 
 	conn = connectToServer(&old_cluster, dbinfo->db_name);