Don't crash on broken GIF images

Broken GIF images could set invalid width and height
values inside the image, leading to Qt creating a null
QImage for it. In that case we need to abort decoding
the image and return an error.

Initial patch by Rich Moore.

Task-number: QTBUG-38367
Change-Id: Id82a4036f478bd6e49c402d6598f57e7e5bb5e1e
Security-advisory: CVE-2014-0190
Reviewed-by: Richard J. Moore <rich@kde.org>