Check if signature from license file is kind of NSData class

Check if `signature` from license file is kind of `NSData` class to prevent an exception when calling `-[signature bytes]`:  

      if (RSA_public_decrypt([signature length], [signature bytes], checkDigest, rsaKey, RSA_PKCS1_PADDING) != 20)
                return nil;

This exception causes `-dictionaryForLicenseData` method to fail, so application's behavior is undefined.