On the X11 backend the cursor position might be outside the output
window so no output is returned leading to the assert to trigger. Use
sane fallback instead of crashing.
When e.g. running rootston under X11 it would otherwise crash when
closing rootston like:
#0 0x00007f0197da7327 in wl_list_remove () at /usr/lib/x86_64-linux-gnu/libwayland-server.so.0
#1 0x000056306fcee7fb in handle_destroy (listener=0x5630723a2948, data=0x5630723a5d20) at ../rootston/layer_shell.c:273
#2 0x00007f019800a552 in wlr_signal_emit_safe (signal=0x5630723a5e30, data=0x5630723a5d20) at ../util/signal.c:29
#3 0x00007f0197fef808 in layer_surface_destroy (surface=0x5630723a5d20) at ../types/wlr_layer_shell.c:169
#4 0x00007f0197ff0001 in client_handle_destroy (resource=0x56307229c4c0) at ../types/wlr_layer_shell.c:371
#5 0x00007f0197da2f30 in () at /usr/lib/x86_64-linux-gnu/libwayland-server.so.0
#6 0x00007f0197da77f9 in () at /usr/lib/x86_64-linux-gnu/libwayland-server.so.0
#7 0x00007f0197da301d in wl_client_destroy () at /usr/lib/x86_64-linux-gnu/libwayland-server.so.0
#8 0x00007f0197da30d8 in () at /usr/lib/x86_64-linux-gnu/libwayland-server.so.0
#9 0x00007f0197da4c12 in wl_event_loop_dispatch () at /usr/lib/x86_64-linux-gnu/libwayland-server.so.0
#10 0x00007f0197da344a in wl_display_run () at /usr/lib/x86_64-linux-gnu/libwayland-server.so.0
#11 0x000056306fcef069 in main (argc=3, argv=0x7ffd22032528) at ../rootston/main.c:83
since the output_destroy got already removed in handle_output_destroy.
