Better error message for fundrawtransaction w/ empty vout

Previously this case failed deep in Cwallet::CreateTransaction() with
the error message "Transaction amounts must be positive"

src/wallet/rpcwallet.cpp

@@ -2418,6 +2418,9 @@ UniValue fundrawtransaction(const UniValue& params, bool fHelp)
     if (!DecodeHexTx(origTx, params[0].get_str()))
         throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed");
 
+    if (origTx.vout.size() == 0)
+        throw JSONRPCError(RPC_INVALID_PARAMETER, "TX must have at least one output");
+
     bool includeWatching = false;
     if (params.size() > 1)
         includeWatching = params[1].get_bool();