net: Have LookupNumeric return a CService directly

Also fix up a few small issues:
- Lookup with "badip:port" now sets the port to 0
- Don't allow assert to have side-effects

src/qt/optionsdialog.cpp

@@ -327,8 +327,7 @@ QValidator::State ProxyAddressValidator::validate(QString &input, int &pos) cons
 {
     Q_UNUSED(pos);
     // Validate the proxy
-    CService serv;
-    LookupNumeric(input.toStdString().c_str(), serv, 9050);
+    CService serv(LookupNumeric(input.toStdString().c_str(), 9050));
     proxyType addrProxy = proxyType(serv, true);
     if (addrProxy.IsValid())
         return QValidator::Acceptable;