crypto: check SecureContext existence

Fix:

	*** CID 179169:  Null pointer dereferences  (NULL_RETURNS)
	/src/node_crypto.cc: 1353 in node::crypto::SecureContext::SetClientCertEngine(const v8::FunctionCallbackInfo<v8::Value> &)()
	1347
	1348       // SSL_CTX_set_client_cert_engine does not itself support multiple
	1349       // calls by cleaning up before overwriting the client_cert_engine
	1350       // internal context variable.
	1351       // Instead of trying to fix up this problem we in turn also do not
	1352       // support multiple calls to SetClientCertEngine.
	>>>     CID 179169:  Null pointer dereferences  (NULL_RETURNS)
	>>>     Dereferencing a null pointer "sc".
	1353       if (sc->client_cert_engine_provided_) {
	1354         return env->ThrowError(
	1355             "Multiple calls to SetClientCertEngine are not allowed");
	1356       }
	1357
	1358       const node::Utf8Value engine_id(env->isolate(), args[0]);

Author: sam-github

src/node_crypto.cc

@@ -1342,6 +1342,7 @@ void SecureContext::SetClientCertEngine(
   CHECK(args[0]->IsString());
 
   SecureContext* sc = Unwrap<SecureContext>(args.This());
+  CHECK_NE(sc, nullptr);
 
   MarkPopErrorOnReturn mark_pop_error_on_return;