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.

src/shared/tpm2-util.c

@@ -2487,6 +2487,7 @@ int tpm2_util_pbkdf2_hmac_sha256(const void *pass,
          */
         static const uint8_t block_cnt[] = { 0, 0, 0, 1 };
 
+        assert (salt);
         assert (saltlen > 0);
         assert (saltlen <= (SIZE_MAX - sizeof(block_cnt)));
         assert (passlen > 0);