interfaces: fix crash report

(cherry picked from commit de2b543d)

src/www/interfaces_assign.php

@@ -186,20 +186,20 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
 
             unset($config['interfaces'][$id]);  /* delete the specified OPTn or LAN*/
 
-            if (is_array($config['dhcpd']) && is_array($config['dhcpd'][$id])) {
+            if (isset($config['dhcpd'][$id])) {
                 unset($config['dhcpd'][$id]);
                 services_dhcpd_configure();
             }
-            if (count($config['filter']['rule']) > 0) {
+            if (isset($config['filter']['rule'])) {
                 foreach ($config['filter']['rule'] as $x => $rule) {
-                    if($rule['interface'] == $id) {
+                    if ($rule['interface'] == $id) {
                         unset($config['filter']['rule'][$x]);
                     }
                 }
             }
-            if (is_array($config['nat']['rule']) && count($config['nat']['rule']) > 0) {
+            if (isset($config['nat']['rule'])) {
                 foreach ($config['nat']['rule'] as $x => $rule) {
-                    if($rule['interface'] == $id) {
+                    if ($rule['interface'] == $id) {
                         unset($config['nat']['rule'][$x]['interface']);
                     }
                 }