One of the dashboard compilers found a code path in PNGDecode where if
given a buffer too short for a header the info.width and info.height
were not initialized but their values still returned to the calling
function. This case does not really matter since an error is also
returned, but still technically incorrect. Fixed the problem by
initializing the width and height to 0 (which could help anyway if the
error is not properly checked).
