Fix crash when pressing delete button in server browser and no server is selected.

A check for that was there, but was comparing an unsigned variable to
-1, which doesn't work.

src/guiMainMenu.cpp

@@ -1053,7 +1053,7 @@ bool GUIMainMenu::OnEvent(const SEvent& event)
 			}
 			case GUI_ID_SERVERLIST_DELETE: {
 				gui::IGUIListBox *serverlist = (gui::IGUIListBox*)getElementFromId(GUI_ID_SERVERLIST);
-				u16 selected = ((gui::IGUIListBox*)serverlist)->getSelected();
+				s32 selected = ((gui::IGUIListBox*)serverlist)->getSelected();
 				if (selected == -1) return true;
 				ServerList::deleteEntry(m_data->servers[selected]);
 				m_data->servers = ServerList::getLocal();