Fix harfbuzz assertions using patch from Chromium.

Exotic text (e.g. attempting to render a binary file such as .zip
or .exe as a webpage) can trigger an assertion failure in the
Harfbuzz code. Chromium developers also noticed this issue and
committed a patch to fix it:

http://lists.freedesktop.org/archives/harfbuzz/2009-August/000354.html

This patch has not been accepted by upstream, but this (old) version
of Harfbuzz has been abandoned and superceded by Harfbuzz-NG.

Issue #11264

*NOTE:* I've lost this patch during the upgrade to Qt 4.8.5.