tfa: handle incompatible challenge data

tfa: handle incompatible challenge data

by returning default data, in case the challenge data is not parseable.
this allows a new challenge to be started for the userid in question
without manual cleanup.

currently this can be triggered if an ongoing challenge created with
webauthn-rs 0.2.5 is stored in /run and attempted to be read
post-upgrade.

Reported-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>