Fix "warning: binding dereferenced null pointer to reference has undefined behavior"

This shows up under debug builds when testing instantiations.

 warning: binding dereferenced null pointer to reference has
      undefined behavior [-Wnull-dereference]
        DH2 dh(*(SimpleKeyAgreementDomain*)NULLPTR);
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

dh2.cpp

@@ -1,14 +1,52 @@
 // dh2.cpp - originally written and placed in the public domain by Wei Dai
 
 #include "pch.h"
+#include "cryptlib.h"
+#include "misc.h"
 #include "dh2.h"
 
 NAMESPACE_BEGIN(CryptoPP)
 
 #if defined(CRYPTOPP_DEBUG) && !defined(CRYPTOPP_DOXYGEN_PROCESSING)
+struct NullCryptoParameters : public CryptoParameters
+{
+	void AssignFrom(const NameValuePairs &source) {
+	}
+	bool Validate(RandomNumberGenerator &rng, unsigned int level) const {
+		return false;
+	}
+	bool GetVoidValue(const char *name, const std::type_info &valueType, void *pValue) const {
+		return false;
+	}
+};
+
+struct NullSimpleKeyAgreementDomain : public TwoBases<NullCryptoParameters, SimpleKeyAgreementDomain>
+{
+	CryptoParameters & AccessCryptoParameters() {
+		return *this;
+	}
+	unsigned int AgreedValueLength() const {
+		return 1;
+	}
+	unsigned int PrivateKeyLength() const {
+		return 1;
+	}
+	unsigned int PublicKeyLength() const {
+		return 1;
+	}
+	void GeneratePrivateKey(RandomNumberGenerator &rng, byte *privateKey) const {
+	}
+	void GeneratePublicKey(RandomNumberGenerator &rng, const byte *privateKey, byte *publicKey) const {
+	}
+	bool Agree(byte *agreedValue, const byte *privateKey, const byte *otherPublicKey, bool validateOtherPublicKey=true) const {
+		return false;
+	}
+};
+
 void DH2_TestInstantiations()
 {
-	DH2 dh(*(SimpleKeyAgreementDomain*)NULLPTR);
+	NullSimpleKeyAgreementDomain dom;
+	DH2 dh(dom);
 }
 #endif