security: Throw exception when loading on orphaned ValidationPolicyConfig

ValidationPolicyConfig must be assigned to a Validator instance prior to
loading and it is an API violation to load configuration otherwise.

Change-Id: Ib372cd3b6ab7c7ce1e37039f25b97e1d2f1511f9
Refs: #4758
diff --git a/src/security/v2/validation-policy.hpp b/src/security/v2/validation-policy.hpp
index 0f86c20..6921984 100644
--- a/src/security/v2/validation-policy.hpp
+++ b/src/security/v2/validation-policy.hpp
@@ -40,11 +40,6 @@
   using ValidationContinuation = std::function<void(const shared_ptr<CertificateRequest>& certRequest,
                                                     const shared_ptr<ValidationState>& state)>;
 
-  ValidationPolicy()
-    : m_validator(nullptr)
-  {
-  }
-
   virtual
   ~ValidationPolicy() = default;
 
@@ -148,7 +143,7 @@
   }
 
 NDN_CXX_PUBLIC_WITH_TESTS_ELSE_PROTECTED:
-  Validator* m_validator;
+  Validator* m_validator = nullptr;
   unique_ptr<ValidationPolicy> m_innerPolicy;
 };