In WriteOnePNGImage() of the PNG coder at coders/png.c, an improper call to AcquireVirtualMemory() and memset() allows for an out-of-bounds write later when PopShortPixel() from MagickCore/quantum-private.h is called. The patch fixes the calls by adding 256 to rowbytes. An attacker who is able to supply a specially crafted image could affect availability with a low impact to data integrity. This flaw affects ImageMagick versions prior to 6.9.10-68 and 7.0.8-68.