Lexer: Fix offset handling in get_str_from()

As the comment said, the subtraction is bogus for multibyte characters.
Fortunately, we can just use last_pos instead of pos to get the correct
position without any subtraction hackery.
auto merge of #7005 : dotdash/rust/fix_get_str_from, r=bstrie

As the comment said, the subtraction is bogus for multibyte characters.
Fortunately, we can just use last_pos instead of pos to get the correct
position without any subtraction hackery.