String conversions uint256 -> uint256S

If uint256() constructor takes a string, uint256(0) will become
dangerous when uint256 does not take integers anymore (it will go
through std::string(const char*) making a NULL string, and the explicit
keyword is no help).

src/rpcwallet.cpp

@@ -1902,7 +1902,7 @@ Value lockunspent(const Array& params, bool fHelp)
         if (nOutput < 0)
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, vout must be positive");
 
-        COutPoint outpt(uint256(txid), nOutput);
+        COutPoint outpt(uint256S(txid), nOutput);
 
         if (fUnlock)
             pwalletMain->UnlockCoin(outpt);