tinywl: don't crash when there is no keyboard

Running with WLR_BACKENDS=headless, there is no keyboard device.
Avoid crashes like so:

    ../tinywl/tinywl.c:136:2: runtime error: member access within null pointer of type 'struct wlr_keyboard'
    ../tinywl/tinywl.c:136:2: runtime error: member access within null pointer of type 'struct wlr_keyboard'
    AddressSanitizer:DEADLYSIGNAL
    =================================================================
    ==331107==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000120 (pc 0x556ed03e4e99 bp 0x7ffce834bc10 sp 0x7ffce834bbb0 T0)
    ==331107==The signal is caused by a READ memory access.
    ==331107==Hint: address points to the zero page.
        #0 0x556ed03e4e99 in focus_view ../tinywl/tinywl.c:136
        #1 0x556ed03eb3be in xdg_toplevel_map ../tinywl/tinywl.c:603
        #2 0x7f75d6f768db in wlr_signal_emit_safe ../util/signal.c:29
        #3 0x7f75d6e9cac7 in xdg_surface_role_commit ../types/xdg_shell/wlr_xdg_surface.c:315
        #4 0x7f75d6eb6944 in surface_commit_state ../types/wlr_compositor.c:466
        #5 0x7f75d6eb7b02 in surface_handle_commit ../types/wlr_compositor.c:523
        #6 0x7f75d5714d49  (/usr/lib/libffi.so.8+0x6d49)
        #7 0x7f75d5714266  (/usr/lib/libffi.so.8+0x6266)
        #8 0x7f75d68cb322  (/usr/lib/libwayland-server.so.0+0xd322)
        #9 0x7f75d68c65cb  (/usr/lib/libwayland-server.so.0+0x85cb)
        #10 0x7f75d68c91c9 in wl_event_loop_dispatch (/usr/lib/libwayland-server.so.0+0xb1c9)
        #11 0x7f75d68c6d36 in wl_display_run (/usr/lib/libwayland-server.so.0+0x8d36)
        #12 0x556ed03eef55 in main ../tinywl/tinywl.c:905
        #13 0x7f75d5d2330f in __libc_start_call_main (/usr/lib/libc.so.6+0x2d30f)
        #14 0x7f75d5d233c0 in __libc_start_main@GLIBC_2.2.5 (/usr/lib/libc.so.6+0x2d3c0)
        #15 0x556ed03e46e4 in _start (/home/simon/src/wlroots/build/tinywl/tinywl+0x136e4)

0 files changed, 0 insertions, 0 deletions