bug#23570: 25.1.50; Cursor motion error with visual-line-mode

[Chong Yidong]

> Thanks, I've pushed a fix to the master branch (commit 99848b3).
> Please test.
>
> (I think this change is too low-level to be safe for the emacs-25
> branch at this late stage of pretest.)

Thanks, verified that the bug seems to be fixed now.

FWIW, I think the fix ought to be considered for the emacs-25 branch,
since (i) it is a regression relative to Emacs 24.4, and (ii) it is
quite simple to trigger on natural-language text with long lines, which
should be considered a common use-case.