fix(wwan): null members after unref and guard destructor to avoid double-free

The destructor unconditionally unref'd current_modem, manager and
connection, but the constructor can leave them NULL or already-unref'd:

- On the mm_manager_new_sync failure path the ctor unref'd connection
  without nulling it, so the dtor unref'd it a second time -> double-free.
- On the g_bus_get_sync failure path all three stay NULL, and in the
  common no-WWAN-hardware case current_modem is NULL, so the dtor ran
  g_object_unref(NULL) -> G_IS_OBJECT assertion criticals.

Use g_clear_object() in the failing ctor path (unref + null) and in the
destructor (NULL-safe unref + null). Teardown is now safe for every ctor
outcome (bus fail, MM fail, no modem, normal), and a normal run still
unrefs each owned ref exactly once.

src/modules/wwan.cpp

@@ -43,7 +43,7 @@ waybar::modules::Wwan::Wwan(const std::string& id, const Json::Value& config)
   if (error) {
     spdlog::error("Failed to create ModemManager proxy: " + std::string(error->message));
     g_error_free(error);
-    g_object_unref(connection);
+    g_clear_object(&connection);
     return;
   }
 
@@ -302,7 +302,7 @@ auto waybar::modules::Wwan::update() -> void {
 }
 
 waybar::modules::Wwan::~Wwan() {
-  g_object_unref(current_modem);
-  g_object_unref(manager);
-  g_object_unref(connection);
+  g_clear_object(&current_modem);
+  g_clear_object(&manager);
+  g_clear_object(&connection);
 }