xwayland: Handle NULL xwl_seat in xwl_seat_can_emulate_pointer_warp

This can happen e.g. with weston's headless backend.
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
(cherry picked from commit e33453f9)

hw/xwayland/xwayland-input.c

@@ -2783,11 +2783,16 @@ xwl_seat_create_pointer_warp_emulator(struct xwl_seat *xwl_seat)
 static Bool
 xwl_seat_can_emulate_pointer_warp(struct xwl_seat *xwl_seat)
 {
-    struct xwl_screen *xwl_screen = xwl_seat->xwl_screen;
+    struct xwl_screen *xwl_screen;
+
+    if (!xwl_seat)
+        return FALSE;
 
     if (!xwl_seat->pointer)
         return FALSE;
 
+    xwl_screen = xwl_seat->xwl_screen;
+
     if (!xwl_screen->relative_pointer_manager)
         return FALSE;