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.

Backport of Id82a4036f478bd6e49c402d6598f57e7e5bb5e1e from Qt 5

Task-number: QTBUG-38367
Change-Id: I0680740018aaa8356d267b7af3f01fac3697312a
Security-advisory: CVE-2014-0190
Reviewed-by: Richard J. Moore <rich@kde.org>
Fix MacOS build with SDK 10.7, XCode 4.3 and above

A hardcoded path condition failed and thus the dependency CoreWLAN
was not added. This caused missing symbols.

This patch is not necessary for Qt5 as only SDKs 10.6+ are supported.

Task-number: QTBUG-24361
Change-Id: I273a2be8d2bd7c72e8d7a1eed7c3e62a380e237a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>