encoding-guesser: Fix bug in parsing fallback encodings.
The encoding guesser originally used UTF-8 as a fallback in situations
where it had already figured out that UTF-8 couldn't be right. Commit
d6c75296 (encoding-guesser: Fall back to windows-1252 when UTF-8 can't be
right.) fixed this, by falling back to window-1252 when UTF-8 was known to
be wrong, but it also introduced a bug where this change to the fallback
encoding occurred even when UTF-8 had not be determined to be wrong. This
commit fixes that problem.
Reported by John Darrington.