[Wallet] Add a check on zPIV spend to avoid a segfault

When zPIV coins are selected and their total value is less than the
target value of the payment, there was nothing to see it before
commiting the transaction. At which point the function would crap out
with an exception which QT would fail to handle, leading to a
segmentation fault. This new check avoids this situation.

src/wallet.cpp

@@ -4677,7 +4677,13 @@ bool CWallet::CreateZerocoinSpendTransaction(CAmount nValue, int nSecurityLevel,
             CScript scriptZerocoinSpend;
             CScript scriptChange;
             CAmount nChange = nValueSelected - nValue;
-            if (nChange && !address) {
+
+            if (nChange < 0) {
+                receipt.SetStatus(_("Selected coins value is less than payment target"), nStatus);
+                return false;
+            }
+
+            if (nChange > 0 && !address) {
                 receipt.SetStatus(_("Need address because change is not exact"), nStatus);
                 return false;
             }