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.
