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.

(cherry picked from commit 879243e370)

xwayland/xwayland.c

@@ -42,6 +42,9 @@ static void handle_server_start(struct wl_listener *listener, void *data) {
 static void xwayland_mark_ready(struct wlr_xwayland *xwayland) {
 	assert(xwayland->server->wm_fd[0] >= 0);
 	xwayland->xwm = xwm_create(xwayland, xwayland->server->wm_fd[0]);
+	// xwm_create takes ownership of wm_fd[0] under all circumstances
+	xwayland->server->wm_fd[0] = -1;
+
 	if (!xwayland->xwm) {
 		return;
 	}