Fixes #152 - Ensure chars read before special processing in lexer if( finish )

Also removed check that characters were read from the default case of the
switch statement (lexer.c:492) in order to remove redundant checks.

This fix can help prevent incorrect tokens from being returned when lexing the
end of the final buffer.
Merge pull request #152 from caitp/issue-152

Only enter `finish` branch in `webvtt_lex` if characters have been read.