Fix startup crash loading saved LAN printers

load_local_machines_from_config() iterated a reference to the live
m_local_machines map while erase_local_machine() erased from it for
printers without access rights, invalidating the range-for iterator
(use-after-free on ++it). Iterate a copy instead, as the code did
before commit 5028a5000e.

src/slic3r/GUI/DeviceCore/DevManager.cpp

@@ -36,7 +36,7 @@ namespace Slic3r
         AppConfig* config = GUI::wxGetApp().app_config;
         if (!config)
             return;
-        const auto& local_machines = config->get_local_machines();
+        const auto local_machines = config->get_local_machines();
         for (auto& it : local_machines) {
             const auto& m = it.second;
             if (localMachineList.count(m.dev_id))