Check return value of XOpenDisplay().

Instead of calling XGetXCBConnection() blindly, check XOpenDisplay()'s
return value to avoid a possible segfault in the former. That happens if
$DISPLAY is set, but if that display isn't available.

Signed-off-by: Cyril Brulebois <kibi@debian.org>

compositor/compositor-x11.c

@@ -553,6 +553,10 @@ x11_compositor_create(struct wl_display *display, int width, int height)
 	memset(c, 0, sizeof *c);
 
 	c->dpy = XOpenDisplay(NULL);
+
+	if (c->dpy == NULL)
+		return NULL;
+
 	c->conn = XGetXCBConnection(c->dpy);
 
 	if (xcb_connection_has_error(c->conn))