libyaml heap overflow resulting in possible code execution

libyaml was prone to a heap overflow that could result in
arbitrary code execution. Pkg uses libyaml to parse
the package manifests in some cases. Pkg also used libyaml
to parse the remote repository until 1.2.
RedHat Product Security Team reports on libyaml:

A heap-based buffer overflow flaw was found in the way libyaml
parsed YAML tags. A remote attacker could provide a
specially-crafted YAML document that, when parsed by an application
using libyaml, would cause the application to crash or, potentially,
execute arbitrary code with the privileges of the user running the
application.