If the compositor sends a keymap that fails on "compilation",
xkb_keymap_new_from_string() returns NULL, which makes xkb_state_new()
crash when assuming there is a keymap.
In these cases, gdk must remain with a xkb_state to handle modifiers/keys
properly, so warn about the invalid keymap string, and keep the previous
keymap (currently initialized to "us")
https://bugzilla.gnome.org/show_bug.cgi?id=735389