createkeyboard: do not segfault if xkb_keymap_new_from_names returns NULL

[link2xt]

Passing NULL to wlr_keyboard_set_keymap results in a segfault.
Example:

  Thread 1 "dwl" received signal SIGSEGV, Segmentation fault.
  0x00007ffff7e49b64 in xkb_keymap_ref () from /usr/lib/libxkbcommon.so.0
  (gdb) bt
  #0  0x00007ffff7e49b64 in xkb_keymap_ref () at /usr/lib/libxkbcommon.so.0
  #1  0x00007ffff7f06389 in wlr_keyboard_set_keymap () at /usr/lib/libwlroots.so.11
  #2  0x000055555555bc54 in createkeyboard ()
  #3  0x000055555555c283 in inputdevice ()
  #4  0x00007ffff7e8101e in wl_signal_emit_mutable () at /usr/lib/libwayland-server.so.0
  #5  0x00007ffff7e8101e in wl_signal_emit_mutable () at /usr/lib/libwayland-server.so.0
  #6  0x00007ffff7edb52c in  () at /usr/lib/libwlroots.so.11
  #7  0x00007ffff7ee44b6 in  () at /usr/lib/libwlroots.so.11
  #8  0x000055555555fe66 in main ()

[link2xt]

My problem was that XKB_DEFAULT_LAYOUT was set to dvorak (this should be us and dvorak should go into XKB_DEFAULT_LAYOUT), but this should not result in a NULL pointer dereference.

[sevz17]

Thanks!