Parse user print info on the UI thread to prevent heap corruption (#119)

get_user_print_info()'s HTTP fetch can run on a worker thread (e.g. BindJob),
but parse_user_print_info() mutates userMachineList (insert/erase/delete
MachineObject). on_machine_alive (SSDP) mutates the same maps on the UI thread
without locking, so parsing off-thread races the map and frees MachineObjects
out from under it -> heap corruption.

Keep all device-list mutation on the UI thread: parse inline when already on
the main thread, otherwise marshal via CallAfter so it stays serialized with
on_machine_alive.

src/slic3r/GUI/DeviceCore/DevManager.cpp

@@ -851,7 +851,9 @@ namespace Slic3r
         int result = m_agent->get_user_print_info(&http_code, &body, provider);
         if (result == 0)
         {
-            parse_user_print_info(body);
+            // parse_user_print_info and on_machine_alive (SSDP for discovery) both mutate the same userMachineList map.
+            // on_machine_alive mutates the map on the UI thread, do the same for parse_user_print_info.
+            Slic3r::GUI::wxGetApp().CallAfter([this, body]() { parse_user_print_info(body); });
         }
     }