layer-shell: Don't dereference gone output
otherwise we might crash on output unplug like
==11985==Hint: address points to the zero page.
#0 0x55ef58999dd7 in handle_unmap ../src/layer_shell.c:354
#1 0x7f82b6f6f618 in wlr_signal_emit_safe ../subprojects/wlroots/util/signal.c:29
#2 0x7f82b6f4613e in layer_surface_unmap ../subprojects/wlroots/types/wlr_layer_shell_v1.c:162
#3 0x7f82b6f47b8b in wlr_layer_surface_v1_close ../subprojects/wlroots/types/wlr_layer_shell_v1.c:249
#4 0x7f82b6f6f618 in wlr_signal_emit_safe ../subprojects/wlroots/util/signal.c:29
#5 0x7f82b6eddd51 in drm_connector_cleanup ../subprojects/wlroots/backend/drm/drm.c:1446
#6 0x7f82b6ee32c1 in scan_drm_connectors ../subprojects/wlroots/backend/drm/drm.c:1238
#7 0x7f82b6f6f618 in wlr_signal_emit_safe ../subprojects/wlroots/util/signal.c:29
#8 0x7f82b6ef4e0b in udev_event ../subprojects/wlroots/backend/session/session.c:52
#9 0x7f82b68aeb71 in wl_event_loop_dispatch (/usr/lib/x86_64-linux-gnu/libwayland-server.so.0+0xab71)
...
Edited by Guido Gunther