Fix favicon warning

https://github.com/FreshRSS/FreshRSS/issues/1445

lib/Favicon/DataAccess.php

@@ -16,7 +16,7 @@ class DataAccess {
 	public function retrieveHeader($url) {
 	    $this->set_context();
 		$headers = @get_headers($url, 1);
-		return $headers ? array_change_key_case($headers) : array();
+		return is_array($headers) ? array_change_key_case($headers) : array();
 	}
 	
     public function saveCache($file, $data) {