Fix some null pointer dereferences in LDAP auth code

An LDAP URL without a host name such as "ldap://" or without a base DN
such as "ldap://localhost" would cause a crash when reading pg_hba.conf.

If no binddn is configured, an error message might end up trying to print a
null pointer, which could crash on some platforms.

Author: Thomas Munro <thomas.munro@enterprisedb.com>
Reviewed-by: Michael Paquier <michael.paquier@gmail.com>

src/backend/libpq/auth.c

@@ -2520,7 +2520,8 @@ CheckLDAPAuth(Port *port)
 		{
 			ereport(LOG,
 					(errmsg("could not perform initial LDAP bind for ldapbinddn \"%s\" on server \"%s\": %s",
-							port->hba->ldapbinddn, port->hba->ldapserver,
+							port->hba->ldapbinddn ? port->hba->ldapbinddn : "",
+							port->hba->ldapserver,
 							ldap_err2string(r)),
 					 errdetail_for_ldap(ldap)));
 			ldap_unbind(ldap);

src/backend/libpq/hba.c

@@ -1739,8 +1739,10 @@ parse_hba_auth_opt(char *name, char *val, HbaLine *hbaline,
 			return false;
 		}
 
+		if (urldata->lud_host)
 			hbaline->ldapserver = pstrdup(urldata->lud_host);
 		hbaline->ldapport = urldata->lud_port;
+		if (urldata->lud_dn)
 			hbaline->ldapbasedn = pstrdup(urldata->lud_dn);
 
 		if (urldata->lud_attrs)