Zend_Validate_Hostname allows "0" subdomain

Fixes #612

library/Zend/Validate/Hostname.php

@@ -1459,7 +1459,7 @@ class Zend_Validate_Hostname extends Zend_Validate_Abstract
                     $check = 0;
                     foreach ($domainParts as $domainPart) {
                         // If some domain part is empty (i.e. zend..com), it's invalid
-                        if (empty($domainPart)) {
+                        if (empty($domainPart) && $domainPart !== '0') {
                             $this->_error(self::INVALID_HOSTNAME);
                             return false;
                         }

tests/Zend/Validate/HostnameTest.php

@@ -536,4 +536,14 @@ class Zend_Validate_HostnameTest extends PHPUnit_Framework_TestCase
         $validator = new Zend_Validate_Hostname();
         $this->assertTrue($validator->isValid('straße.de'));
     }
+
+    /**
+     * @group GH-612
+     */
+    public function testZeroSubdomain()
+    {
+        $validator = new Zend_Validate_Hostname();
+        $this->assertTrue($validator->isValid('1.pool.ntp.org'));
+        $this->assertTrue($validator->isValid('0.pool.ntp.org'));
+    }
 }