ImageMagick: integer overflows in MNG magnification

ImageMagick is free and open-source software used for editing and manipulating digital images. Prior to versions 6.9.13-27 and 7.1.2-1, the magnified size calculations in ReadOneMNGIMage (in coders/png.c) are unsafe and can overflow, leading to memory corruption. This issue has been patched in versions 6.9.13-27 and 7.1.2-1.