zgv, xzgv -- heap overflow vulnerability

Gentoo reports:

Andrea Barisani of Gentoo Linux discovered xzgv and zgv
allocate insufficient memory when rendering images with
more than 3 output components, such as images using the
YCCK or CMYK colour space. When xzgv or zgv attempt to
render the image, data from the image overruns a heap
allocated buffer.
An attacker may be able to construct a malicious image that
executes arbitrary code with the permissions of the xzgv or
zgv user when attempting to render the image.