also make sure 1 character length is invalid

Author: joonsp

library/Xi/Algorithm/Luhn.php

@@ -42,6 +42,10 @@ public function generate($number)
      */
     public function validate($number)
     {
+        if (strlen($number) < 2) {
+            return false;
+        }
+
         $original = substr($number, 0, strlen($number) - 1);
 
         return $this->generate($original) === $number;

tests/Xi/Tests/Algorithm/LuhnTest.php

@@ -88,6 +88,7 @@ public function luhnInvalidProvider()
             array(799273983),
             array(1231),
             array(4565),
+            array(1),
         );
     }
 }