[PATCH] Frontend: Avoid some UB by checking a FileChange's validity before a call

[Justin Bogner]

If a FileChange has an invalid file, the FileType won't be initialized,
so it's undefined to pass it into Process like this. We immediately bail
out because the FileID is invalid in that case, so we can avoid the
issue by checking that first.

The alternative is to initialize FileType in FileChange's constructor,
but it's not clear if there's a reasonable default. Should I do that
instead, or is this good to commit?

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