(legacy) fix crashreport PHP Warning: Illegal string offset 'wan' in...

(legacy) fix crashreport PHP Warning:  Illegal string offset 'wan' in /usr/local/www/services_dhcpv6.php on line 57

(cherry picked from commit 4f0a083e)

src/www/services_dhcpv6.php

@@ -54,8 +54,8 @@ $iflist = array_merge($iflist, get_configured_pppoe_server_interfaces());
 if (!$if || !isset($iflist[$if])) {
 	foreach ($iflist as $ifent => $ifname) {
 		$oc = $config['interfaces'][$ifent];
-		if ((is_array($config['dhcpdv6'][$ifent]) && !isset($config['dhcpdv6'][$ifent]['enable']) && !(is_ipaddrv6($oc['ipaddrv6']) && (!is_linklocal($oc['ipaddrv6'])))) ||
-			(!is_array($config['dhcpdv6'][$ifent]) && !(is_ipaddrv6($oc['ipaddrv6']) && (!is_linklocal($oc['ipaddrv6'])))))
+		if ((!empty($config['dhcpdv6'][$ifent]) && !isset($config['dhcpdv6'][$ifent]['enable']) && !(is_ipaddrv6($oc['ipaddrv6']) && (!is_linklocal($oc['ipaddrv6'])))) ||
+			(empty($config['dhcpdv6'][$ifent]) && !(is_ipaddrv6($oc['ipaddrv6']) && (!is_linklocal($oc['ipaddrv6'])))))
 			continue;
 		$if = $ifent;
 		break;