textiter: fix bug in FindLogAttrFunc functions

attrs[len] is the last PangoLogAttr available, at the iter position after the
last character of the line.

For a line in the middle or the start of the buffer, the '\n' is taken
into account by 'len'. For example the is_word_end is generally reached
before the '\n', not after. But for the last line in the buffer, where
there is no trailing '\n', it is important to test until attrs[len].

The bug didn't occur before because find_by_log_attrs() worked directly
on the iter passed as the function argument. But now it is no longer the
case.