refactor: Make const refs vars where applicable

This avoids initializing variables with the copy-constructor of a
non-trivially copyable type.

src/bitcoin-tx.cpp

@@ -596,7 +596,7 @@ static void MutateTxSign(CMutableTransaction& tx, const std::string& flagStr)
     UniValue prevtxsObj = registers["prevtxs"];
     {
         for (unsigned int previdx = 0; previdx < prevtxsObj.size(); previdx++) {
-            UniValue prevOut = prevtxsObj[previdx];
+            const UniValue& prevOut = prevtxsObj[previdx];
             if (!prevOut.isObject())
                 throw std::runtime_error("expected prevtxs internal object");