fix bug with multiple minimized interceptions vs terror ships

handling for null pointers on UFOs after they become terror sites.

the "aliens terrorize <x>" popup is enough, not setting the ufo to undetected
removes the "ufo lost" popup and causes craft to return to base on their own
without popping up a "redirect craft" window.
don't send skyrangers back to base.

Extending the previous commit a little, this adds handling for landing craft
sent to intercept a landing terror ship.