xwm: Fix double-close

When an FD is passed to xcb_connect_to_fd(), xcb takes ownership of that
FD and is responsible for closing it, which it does when
xcb_disconnect() is called.  But the xwayland handler code also keeps a
copy of the FD and closes it via safe_close() in
server_finish_process().

This double-close can cause all sorts of problems if another part of
wlroots allocates another FD between the two closes - the latter close
will close the wrong FD and things go horribly wrong (in my case leading
to use-after-free and segfaults).

Fix this by setting wm_fd[0]=-1 after calling xwm_create(), and ensuring
that xwm_create() closes the FD if startup errors occur.

include/xwayland/xwm.h

@@ -164,6 +164,7 @@ struct wlr_xwm {
 	struct wl_listener drop_focus_destroy;
 };
 
+// xwm_create takes ownership of wm_fd and will close it under all circumstances.
 struct wlr_xwm *xwm_create(struct wlr_xwayland *wlr_xwayland, int wm_fd);
 
 void xwm_destroy(struct wlr_xwm *xwm);