weston crashes in weston_desktop_xdg_toplevel_protocol_set_fullscreen() of xdg-shell.c if disconnect the screen just when calling set fullscreen

Analysis

when this issue happens, the destroy_pending is set
after dumping the calling stack when SEGV, it shows weston_head_from_resource(output_resource) return a NULL head
the following patch can fix this issue, but I suppose other implementations like xdg-shell may also have this issue.

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