shell: fix segfault in fullscreen binding

Commit 9aa8ce69 forgot to set shsurf->fullscreen_output in
fullscreen_binding(), causing segfault when fullscreening using key
bindings. This patch fixes that.

https://bugs.freedesktop.org/show_bug.cgi?id=79828
Signed-off-by: Boyan Ding <stu_dby@126.com>

desktop-shell/shell.c

@@ -4193,6 +4193,7 @@ fullscreen_binding(struct weston_seat *seat, uint32_t time, uint32_t button, voi
 
 	shsurf->state_requested = true;
 	shsurf->requested_state.fullscreen = !shsurf->state.fullscreen;
+	shsurf->fullscreen_output = shsurf->output;
 	send_configure_for_surface(shsurf);
 }