keyboard: Fix SIGSEGV that showed up in out-of-memory fuzzing

keyboard: Fix SIGSEGV that showed up in out-of-memory fuzzing

Stack trace:

    #0  xkb_keymap_ref (keymap=keymap@entry=0x0) at ../libxkbcommon/src/keymap.c:61
    #1  0x00007f53a344ab99 in wlr_keyboard_set_keymap (kb=kb@entry=0x5571af8cb9a0, keymap=keymap@entry=0x0)
        at ../types/wlr_keyboard.c:174
    #2  0x00005571ade057e0 in keyboard_init (seat=0x7ffca0389680) at ../src/keyboard.c:229
    #3  seat_init (server=0x7ffca0389570) at ../src/seat.c:307
    #4  server_init (server=0x7ffca0389570) at ../src/server.c:308