Use the correct length value for input salt

Use the correct length value for input salt

In this function the salt can be either a zero buffer of exactly mdlen
length, or an arbitrary salt of prevsecretlen length.
Although in practice OpenSSL will always pass in a salt of mdlen size
bytes in the current TLS 1.3 code, the openssl kdf command can pass in
arbitrary values (I did it for testing), and a future change in the
higher layer code could also result in unmatched lengths.

If prevsecretlen is > mdlen this will cause incorrect salt expansion, if
prevsecretlen < mdlen this could cause a crash or reading random
information. Inboth case the generated output would be incorrect.

Signed-off-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25579)

(cherry picked from commit 5c91f70ba8f07eeeb02b6c285479e4482443a6fe)