Add check that vpubs are not both nonzero and test it.

src/gtest/test_checktransaction.cpp

@@ -0,0 +1,26 @@
+#include <gtest/gtest.h>
+
+#include "main.h"
+#include "primitives/transaction.h"
+#include "consensus/validation.h"
+
+TEST(checktransaction_tests, check_vpub_not_both_nonzero) {
+    CMutableTransaction tx;
+    tx.nVersion = 2;
+
+    {
+        // Ensure that values within the pour are well-formed.
+        CMutableTransaction newTx(tx);
+        CValidationState state;
+        state.SetPerformPourVerification(false); // don't verify the snark
+
+        newTx.vpour.push_back(CPourTx());
+
+        CPourTx *pourtx = &newTx.vpour[0];
+        pourtx->vpub_old = 1;
+        pourtx->vpub_new = 1;
+
+        EXPECT_FALSE(CheckTransaction(newTx, state));
+        EXPECT_EQ(state.GetRejectReason(), "bad-txns-vpubs-both-nonzero");
+    }
+}