bug#13817: 24.3.50; Disabling whitespace-mode that was enabled in sgml-mode-hook breaks font-lock

[Dmitry Gutov]

Revno 111881 fixed the obvious problem with whitespace-mode, here's a
less obvious one.

This seems to be localized to sgml-mode and its derivatives so far:
js-mode, ruby-mode, etc are not affected.

1. (add-hook 'sgml-mode-hook 'whitespace-mode)
2. Open a file in sgml-mode or html-mode, or create a buffer and turn
either mode manually. whitespace-mode will be enabled.
3. M-x whitespace-mode => it's disabled, font-lock is gone, the value of
font-lock-keywords is (t nil).

On the surface, the reason for this is that when whitespace-mode is being
turned on in the hook, font-lock-keywords is still nil, and so nil is
saved in whitespace-font-lock-keywords.

In GNU Emacs 24.3.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.6.0)
 of 2013-02-26 on vbx
Bzr revision: 111882 eggert@cs.ucla.edu-20130226030908-c65ivknn1bm5hjru
Windowing system distributor `The X.Org Foundation', version 11.0.11300000
System Description:     Ubuntu 12.10