[Bricksmith] MPD file handling crash

[Travis Cobbs]

Someone reported a crash, and after investigation it was determined that his development parts had one file as a moved-to that referred to another file that was in turn a reference to the moved-to part. So I added the code as a direct result of a crash in the field. The currently released LDView has a bug in its detection of circular references such that if one of the sub-files of an MPD refers back to the main subfile of the MPD, it still crashes. I've fixed that in my development code tree, but the reason I found out about the problem is that a different user reported that crash.