textiter: fix bug in find_visible_by_log_attrs()

find_by_log_attrs() can return true only in this case:
return moved && !gtk_text_iter_is_end (arg_iter);

So if the iter moved (i.e. something has been found), but is the end
iter, find_by_log_attrs() returns false.

Now the same checks are made in find_visible_by_log_attrs(). The public
functions using find_visible_by_log_attrs() say in their documentation
that false is returned for the end iter, hence the check with
gtk_text_iter_is_end().

https://bugzilla.gnome.org/show_bug.cgi?id=618852