Fix potential array access on non array

See agnostack#185 and
agnostack#181

src/app/code/community/Zendesk/Zendesk/Model/Api/Tickets.php

@@ -82,7 +82,9 @@ public function forOrder($orderIncrementId)
             ]
         );
 
-        if(count($response['results'])) {
+        // Check if $response['results'] is set and is an array before calling count()
+        // See https://github.com/agnostack/magento_extension/pull/185
+        if (isset($response['results']) && is_array($response['results']) && count($response['results'])) {
             return $response['results'];
         } else {
             return false;