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>