fix a setjmp() loop in pngread
fix a setjmp() loop in pngread

see

https://github.com/jcupitt/libvips/issues/175

stupid setjmp()

wishabi/libvips,mcanthony/libvips,jcupitt/libvips,leiyangyou/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,leiyangyou/libvips,lovell/libvips,mcanthony/libvips,leiyangyou/libvips,lovell/libvips,lovell/libvips,wishabi/libvips,mcanthony/libvips,leiyangyou/libvips,wishabi/libvips,wishabi/libvips,lovell/libvips,mcanthony/libvips