pk: Allow out == NULL in canonicalize_uuid()
pk: Allow out == NULL in canonicalize_uuid()

- Allow canonicalize_uuid() to be called with a NULL second argument, which
validates the UUID specified in the first argument but doesn't copy it
anywhere

- Return PK_NOMEM instead of PK_INVALID from canonicalize_uuid() when
appropriate

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr