input-capture/session: Disconnect on_keymap_changed on session finalize

When Input Capture was enabled on Input Leap server startup and then
finalized when Input Leap server was stopped, switching keymap was
still triggering its on_keymap_changed callback, but on a freed session
thus triggering use after free a segfault.

Fixes: 2fb3bdf77 - input-capture: Hook up capturing of events to active session
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/3360
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4257>

src/backends/meta-input-capture-session.c

@@ -1188,6 +1188,10 @@ static void
 meta_input_capture_session_finalize (GObject *object)
 {
   MetaInputCaptureSession *session = META_INPUT_CAPTURE_SESSION (object);
+  MetaBackend *backend =
+    meta_dbus_session_manager_get_backend (session->session_manager);
+
+  g_signal_handlers_disconnect_by_func (backend, on_keymap_changed, session);
 
   g_clear_pointer (&session->barriers, g_hash_table_unref);