Check if the country-region combination exists.

Check that a given country and region combination for an IP exists in
$crnMap.
Fix issue reported by @xifrin in
https://github.com/dlage/yii-geoip/pull/2

GeoIP/CRN.php

@@ -4513,7 +4513,7 @@ class GeoIP_CRN {
    * @return string Metro region name.
    */
   public static function getRegionName($countryCode, $regionCode) {
-    if ($countryCode === null || $regionCode === null) {
+    if ($countryCode === null || $regionCode === null || !isset(self::$crnMap[$countryCode][$regionCode])) {
       return null;
     }
     return self::$crnMap[$countryCode][$regionCode];