Segfault on the-compiler.org in QtWebKit with old HarfBuzz

When opening http://www.the-compiler.org/ with the example browser application with `QT_HARFBUZZ=old` set, I get a segfault.

When I set `QT_HARFBUZZ=new` or downgrade to Qt 5.2 (with harfbuzz old/new), the crash doesn't happen.

The crash seems to happen often but not always, and never so far when running under gdb. For some odd reason I also can't get a coredump though configured correctly. If you can't reproduce let me know, then I'll investigate further.