PNG: fix memory leak when error takes early exit (#3543)
PNG: fix memory leak when error takes early exit (#3543)

Found by leak sanitizer.
lgritz/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio