meta_window_move_frame(): fix crash when frame is NULL
When meta_frame_calc_borders() was made to take a NULL frame argument,
a crash was accidentally introduced into meta_window_move_frame().
This partially reverts
8c1b2d5.
https://bugzilla.gnome.org/show_bug.cgi?id=675254
Gbp-Pq: Name 00_meta_window_move_frame-fix-crash-when-frame-is-NULL.patch