snapd non-regular file indefinite blocking read

In snapd versions prior to 2.62, snapd failed to properly check the file
type when extracting a snap. The snap format is a squashfs file-system
image and so can contain files that are non-regular files (such as pipes
or sockets etc). Various file entries within the snap squashfs image
(such as icons etc) are directly read by snapd when it is extracted. An
attacker who could convince a user to install a malicious snap which
contained non-regular files at these paths could then cause snapd to block
indefinitely trying to read from such files and cause a denial of service.