Avoid using atooid for numerical comparisons which arent Oids

The check for the number of roles in the target cluster for an upgrade
selects the existing roles and performs a COUNT(*) over the result.  A
value of one is the expected query result value indicating that only
the install user is present in the new cluster. The result was converted
with the function for converting a string containing an Oid into a numeric,
which avoids potential overflow but makes the code less readable since
it's not actually an Oid at all.

Discussion: https://postgr.es/m/41AB5F1F-4389-4B25-9668-5C430375836C@yesql.se

Author: danielgustafsson

src/bin/pg_upgrade/check.c

@@ -568,7 +568,7 @@ check_is_install_user(ClusterInfo *cluster)
 	 * users might match users defined in the old cluster and generate an
 	 * error during pg_dump restore.
 	 */
-	if (cluster == &new_cluster && atooid(PQgetvalue(res, 0, 0)) != 1)
+	if (cluster == &new_cluster && strcmp(PQgetvalue(res, 0, 0), "1") != 0)
 		pg_fatal("Only the install user can be defined in the new cluster.");
 
 	PQclear(res);