this is an intermittent crash in the real world but under event testing we can
see that the PaintView is informed that the page has been deleted, but doesn't
clear its use of the page if it so happens that there was earlier modification
events that caused the fire-modification-timer to be launched

If there happens to be no such events then the page is correctly cleared

If the timer is active, then the page is not cleared, but eventually the
timer will fire and access the deleted page and fall over and die

minimal change for backporting ease, follow ups now

(cherry picked from commit f8b97599)

Change-Id: I090ad62cc175545902574c93dec7082225d97ab7
Reviewed-on: https://gerrit.libreoffice.org/19551Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>
(cherry picked from commit d6a6261f)