Fix use-after-free in x11 backend during shutdown

The xcb_connection_t instance that is used here comes from
XGetXCBConnection(), is created by XOpenDisplay(), and is owned by the
returned Display*. Calling xcb_disconnect() directly on it leads to
various use-after-frees during shutdown, as reported by valgrind. The
first one of the about 30 errors is:

    Invalid read of size 4
       at 0x71F2051: xcb_take_socket (in /usr/lib64/libxcb.so.1.1.0)
       by 0x78551DD: ??? (in /usr/lib64/libX11.so.6.3.0)
       by 0x7855A14: _XFlush (in /usr/lib64/libX11.so.6.3.0)
       by 0x7858504: _XGetRequest (in /usr/lib64/libX11.so.6.3.0)
       by 0x7838966: XFreeGC (in /usr/lib64/libX11.so.6.3.0)
       by 0x783238B: XCloseDisplay (in /usr/lib64/libX11.so.6.3.0)
       by 0x4E680C2: wlr_x11_backend_destroy (backend.c:333)
       by 0x4E57E94: wlr_backend_destroy (backend.c:39)
       by 0x4E629FB: multi_backend_destroy (backend.c:47)
       by 0x4E62B5A: handle_display_destroy (backend.c:90)
       by 0x50B7E9F: ??? (in /usr/lib64/libwayland-server.so.0.1.0)
       by 0x50B8476: wl_display_destroy (in /usr/lib64/libwayland-server.so.0.1.0)
     Address 0xc14dda0 is 0 bytes inside a block of size 21,152 free'd
       at 0x4C2DD18: free (vg_replace_malloc.c:530)
       by 0x4E680A5: wlr_x11_backend_destroy (backend.c:330)
       by 0x4E57E94: wlr_backend_destroy (backend.c:39)
       by 0x4E629FB: multi_backend_destroy (backend.c:47)
       by 0x4E62B5A: handle_display_destroy (backend.c:90)
       by 0x50B7E9F: ??? (in /usr/lib64/libwayland-server.so.0.1.0)
       by 0x50B8476: wl_display_destroy (in /usr/lib64/libwayland-server.so.0.1.0)
       by 0x40C54E: main (main.c:84)
     Block was alloc'd at
       at 0x4C2EA1E: calloc (vg_replace_malloc.c:711)
       by 0x71F0C60: xcb_connect_to_fd (in /usr/lib64/libxcb.so.1.1.0)
       by 0x71F4BD4: xcb_connect_to_display_with_auth_info (in /usr/lib64/libxcb.so.1.1.0)
       by 0x7854AA1: _XConnectXCB (in /usr/lib64/libX11.so.6.3.0)
       by 0x7845481: XOpenDisplay (in /usr/lib64/libX11.so.6.3.0)
       by 0x4E681B6: wlr_x11_backend_create (backend.c:376)
       by 0x4E580EE: wlr_backend_autocreate (backend.c:99)
       by 0x40C27D: main (main.c:35)

Normally, one would expect this to crash during XCloseDisplay() when
xcb_disconnect() is called again and frees the same data again (glibc would
detect a double free). However, XCloseDisplay() tries to clean up some internal
caches first for which it has to send requests to the X11 server (e.g. the
XFreeGC() above). This fails since the file descriptor was already closed,
which causes an IO error. Xlib's _XDefaultIOError() handles this by printing an
error message and calling exit(1).

Thus, the only symptom of this problem was compositors exiting
mid-shutdown and printing an error message:

    XIO:  fatal IO error 11 (Resource temporarily unavailable) on X server ":0"
          after 6 requests (6 known processed) with 0 events remaining.

Fixes: https://github.com/swaywm/wlroots/issues/745
Signed-off-by: Uli Schlachter <psychon@znc.in>

backend/x11/backend.c

@@ -326,9 +326,6 @@ static void wlr_x11_backend_destroy(struct wlr_backend *backend) {
 	wl_event_source_remove(x11->frame_timer);
 	wlr_egl_finish(&x11->egl);
 
-	if (x11->xcb_conn) {
-		xcb_disconnect(x11->xcb_conn);
-	}
 	if (x11->xlib_conn) {
 		XCloseDisplay(x11->xlib_conn);
 	}
@@ -428,7 +425,6 @@ struct wlr_backend *wlr_x11_backend_create(struct wl_display *display,
 error_event:
 	wl_event_source_remove(x11->event_source);
 error_x11:
-	xcb_disconnect(x11->xcb_conn);
 	XCloseDisplay(x11->xlib_conn);
 	free(x11);
 	return NULL;