Only fallback to ASCII/ANSI parsing if no UTF-8 BOM was present.

The presence of an UTF-8 BOM should indicate that the file actually is UTF-8,
and one invalid character should not be considered as a reason to think
otherwise.

Only if no UTF-8 BOM is present in the file, the auto-detection needs the fallback.
Properly recover after an invalid UTF-8 character.

bValid was never properly set to TRUE after an invalid character,
which was no problem before because UTF-8 parsing was aborted. However,
if parsing is to simply skip invalid characters, this case needs to be
handled properly.
Merge pull request #69 from Nevcairiel/utf8subsimprov

Improve handling of UTF-8 subtitles with BOM.

Never fallback to ASCII/ANSI in that case even if an invalid UTF-8 character is detected.