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>


(cherry picked from commit 60f647df)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1609>