cryptsetup: do not assert when unsealing token without salt

cryptsetup: do not assert when unsealing token without salt

Salt was added in v253. We are not checking whether it was actually found
(non-zero size), so when an old tpm+pin enrollment is opened things go boom.
For good measure, check both the buffer and the size in both places.

Assertion 'saltlen > 0' failed at src/shared/tpm2-util.c:2490, function tpm2_util_pbkdf2_hmac_sha256(). Aborting.