backends/native: Don't crash if keymap is misconfigured

xkb recently gained support for user-specified keymaps, which means we
can no longer assume that the configuration data is necessarily fully
complete or correct; and the configuration language is quite a labyrinth,
so it's easy to get wrong. If setting the keymap fails, leave it in
whatever state it previously had, since that seems preferable to crashing
with a NULL pointer dereference.

Resolves: https://gitlab.gnome.org/GNOME/mutter/-/issues/1555
Signed-off-by: Simon McVittie <smcv@debian.org>
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1605>

src/backends/native/meta-backend-native.c

@@ -284,6 +284,14 @@ meta_backend_native_set_keymap (MetaBackend *backend,
   keymap = xkb_keymap_new_from_names (context, &names, XKB_KEYMAP_COMPILE_NO_FLAGS);
   xkb_context_unref (context);
 
+  if (keymap == NULL)
+    {
+      g_warning ("Unable to load configured keymap: rules=%s, model=%s, layout=%s, variant=%s, options=%s",
+                 DEFAULT_XKB_RULES_FILE, DEFAULT_XKB_MODEL, layouts,
+                 variants, options);
+      return;
+    }
+
   seat = clutter_backend_get_default_seat (clutter_backend);
   meta_seat_native_set_keyboard_map (META_SEAT_NATIVE (seat), keymap);