Initialize variables to fix valgrind warning in pnghandler.

Conditional jump or move depends on uninitialised value(s)
==7986==    at 0x5B838E8: QPngHandlerPrivate::readPngImage(QImage*)
(qpnghandler.cpp:617)

Change-Id: Ie739ca1665ce600426e2816a6229145d814125cb
Reviewed-by: aavit <eirik.aavitsland@digia.com>