Squash some warnings about signed-unsigned comparisons in png.c
Squash some warnings about signed-unsigned comparisons in png.c


Danack/ImageMagick,UniQP/ImageMagick,UniQP/ImageMagick,kwm81/ImageMagick,UniQP/ImageMagick,UniQP/ImageMagick,UniQP/ImageMagick,Danack/ImageMagick,Danack/ImageMagick,kwm81/ImageMagick,kwm81/ImageMagick,Danack/ImageMagick,kwm81/ImageMagick,Danack/ImageMagick,Danack/ImageMagick,kwm81/ImageMagick,kwm81/ImageMagick,UniQP/ImageMagick