rpc/wallet: Workaround older UniValue which returns a std::string temporary for get_str

Author: luke-jr

src/wallet/rpcwallet.cpp

@@ -729,7 +729,8 @@ UniValue getbalance(const JSONRPCRequest& request)
     if (request.params.size() == 0)
         return  ValueFromAmount(pwallet->GetBalance());
 
-    const std::string* account = request.params[0].get_str() != "*" ? &request.params[0].get_str() : nullptr;
+    const std::string& account_param = request.params[0].get_str();
+    const std::string* account = account_param != "*" ? &account_param : nullptr;
 
     int nMinDepth = 1;
     if (request.params.size() > 1)