session-lockdown: don't assume that unparsed contexts are unconfined

In practice, the regex should always match: AppArmor "confinement
strings" appear to always contain a label and mode, except in the
special case "unconfined". However, if this is untrue for whatever
reason, we should log it as an error, not carry on blindly.

Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3637