textiter: fix bug in _gtk_text_btree_get_iter_at_last_toggle()

textiter: fix bug in _gtk_text_btree_get_iter_at_last_toggle()

If the last tag toggle is the end iter, the function returned the wrong
tag toggle.

This resulted in some bugs where the view wasn't relayout/redrawn
correctly.

The function also always returned TRUE, probably because the return
value is used nowhere. But for consistency with
_gtk_text_btree_get_iter_at_first_toggle(), it's better to keep the
return value, and also because otherwise the function would be wrong (it
doesn't always return a tag toggle, if there is none).

https://bugzilla.gnome.org/show_bug.cgi?id=755413