xwayland: Fix a segfault with pointer locking

Xwayland would crash in some circumstances while trying to issue a
pointer locking when the cursor is hidden when there is no seat focus
window set.

The crash signature looks like:

 #0  zwp_pointer_constraints_v1_lock_pointer ()
 #1  xwl_pointer_warp_emulator_lock () at xwayland-input.c:2584
 #2  xwl_seat_maybe_lock_on_hidden_cursor () at xwayland-input.c:2756
 #3  xwl_seat_maybe_lock_on_hidden_cursor () at xwayland-input.c:2765
 #4  xwl_seat_cursor_visibility_changed () at xwayland-input.c:2768
 #5  xwl_set_cursor () at xwayland-cursor.c:245
 #6  miPointerUpdateSprite () at mipointer.c:468
 #7  miPointerDisplayCursor () at mipointer.c:206
 #8  CursorDisplayCursor () at cursor.c:150
 #9  AnimCurDisplayCursor () at animcur.c:220
 #10 ChangeToCursor () at events.c:936
 #11 ActivatePointerGrab () at events.c:1542
 #12 GrabDevice () at events.c:5120
 #13 ProcGrabPointer () at events.c:4908
 #14 Dispatch () at dispatch.c:478
 #15 dix_main () at main.c:276

xwl_pointer_warp_emulator_lock() tries to use the surface from the
xwl_seat->focus_window leading to a NULL pointer dereference when that
value is NULL.

Check that xwl_seat->focus_window is not NULL earlier in the stack in
xwl_seat_maybe_lock_on_hidden_cursor() and return early if not the case
to avoid the crash.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=102474
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

hw/xwayland/xwayland-input.c

@@ -2762,6 +2762,9 @@ xwl_seat_maybe_lock_on_hidden_cursor(struct xwl_seat *xwl_seat)
         !xwl_seat->cursor_confinement_window)
         return FALSE;
 
+    if (!xwl_seat->focus_window)
+        return FALSE;
+
     if (xwl_seat->confined_pointer)
         xwl_seat_destroy_confined_pointer(xwl_seat);