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/httpserver.cpp

@@ -619,7 +619,7 @@ CService HTTPRequest::GetPeer()
         const char* address = "";
         uint16_t port = 0;
         evhttp_connection_get_peer(con, (char**)&address, &port);
-        LookupNumeric(address, peer, port);
+        peer = LookupNumeric(address, port);
     }
     return peer;
 }