Refactor hba_authname

The previous implementation (from 9afffcb833) had an unnecessary check
on the boundaries of the enum which trigtered compile warnings. To clean
it up, move the pre-existing static assert to a central location and
call that.

Reported-By: Erik Rijkers
Reviewed-By: Michael Paquier
Discussion: https://postgr.es/m/1056399262.13159.1617793249020@webmailclassic.xs4all.nl

src/backend/libpq/auth.c

@@ -379,7 +379,7 @@ set_authn_id(Port *port, const char *id)
 		ereport(LOG,
 				errmsg("connection authenticated: identity=\"%s\" method=%s "
 					   "(%s:%d)",
-					   port->authn_id, hba_authname(port), HbaFileName,
+					   port->authn_id, hba_authname(port->hba->auth_method), HbaFileName,
 					   port->hba->linenumber));
 	}
 }