signature-params: Reject RSASSA-PSS params that result in negative salt len

signature-params: Reject RSASSA-PSS params that result in negative salt len

The `salt_len` member in the struct is of type `ssize_t` because we use
negative values for special automatic salt lengths when generating
signatures.  This change ensures that `salt_len` will not overflow the
`len` fields of chunks (`size_t`), which could lead to integer overflows
when validating signatures (see the next commit).

Fixes: a22316520b91 ("signature-params: Add functions to parse/build ASN.1 RSASSA-PSS params")