Add missing variables initialization before requesting png offset

Initialize all values read to sensible default in case reading them
fails.

Qt 5 fix commit 0ab3e290c4808f73d46903a5274929e3833d8d3c

Task-number: QTBUG-32674
Change-Id: I3a120b5673b67f705a4ebec4359aa52e2392492a
Reviewed-by: aavit <eirik.aavitsland@digia.com>