bug#21629: 25.0.50; python.el freezes up around docstrings.

[Eli Zaretskii <eliz@gnu.org>]

> From: Jacob MacDonald <jaccarmac@gmail.com>
> Date: Tue, 06 Oct 2015 04:48:50 +0000
> 
> Begin from a fresh 'emacs -Q'. Create a Python file. Insert a long
> docstring which contains a header line and then a long body which
> includes a few line breaks and spaces in it. The docstring should look
> something like this:
> 
> """test header
> 
> lorem ipsum... more text more text more text
> x10 lines
> ...
> ...
> """
> 
> Save the file and close emacs. Start a fresh 'emacs -Q'. Open dired in
> the directory which contains the Python file you created. Navigate to
> the file and try to open in from dired. Emacs will freeze indefinitely
> and eat all the system resources it can. You can break the cycle by
> pressing C-q many times, but the freeze happens again on every single
> redisplay. If you wait awhile before cancelling the redisplay, you may
> see that fontification has frozen somewhere in the middle of the docstring.

It infloops in python-nav-end-of-statement.  The loop there ends up
one position before EOB, then jumps back to string-start, and so on
and so forth, ad nauseam.

I have no idea what causes this, but I hope Python-mode experts and
perhaps Stefan (due to syntax stuff) will be able to figure this out.