crypto: fix large DH generator validation

Unfortunately, `std::optional<>` implements `operator<` in such a way
that this check will fail for very large generators. Since `bn_g` is
unsigned, if its value does not fit into a single word, we can be
certain that it is at least 2. By only checking the value if it does
indeed fit into a word, the check correctly ignores very large
generators.

Signed-off-by: Tobias Nießen <tniessen@tnie.de>

Author: tniessen

src/crypto/crypto_dh.cc

@@ -196,7 +196,7 @@ void New(const FunctionCallbackInfo<Value>& args) {
 #endif
       return ThrowCryptoError(env, ERR_get_error(), "Invalid generator");
     }
-    if (bn_g.getWord() < 2) {
+    if (bn_g.getWord().has_value() && bn_g.getWord().value() < 2) {
 #ifndef OPENSSL_IS_BORINGSSL
       ERR_put_error(ERR_LIB_DH, 0, DH_R_BAD_GENERATOR, __FILE__, __LINE__);
 #else