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>

src/gui/image/qgifhandler.cpp

@@ -359,6 +359,13 @@ int QGIFFormat::decode(QImage *image, const uchar *buffer, int length,
                     memset(bits, 0, image->byteCount());
                 }
 
+                // Check if the previous attempt to create the image failed. If it
+                // did then the image is broken and we should give up.
+                if (image->isNull()) {
+                    state = Error;
+                    return -1;
+                }
+
                 disposePrevious(image);
                 disposed = false;