Ensure null termination of AFS salts

Ensure null termination of AFS salts

Use krb5int_copy_data_contents_add0 when copying a pa-pw-salt or
pa-afs3-salt value in pa_salt().  If it's an afs3-salt, we're going to
throw away the length and use strcspn in krb5int_des_string_to_key,
which isn't safe if the value is unterminated.

(cherry picked from commit f566fee75f2455d6e5e7ee4fcdf5a0d327808639)

ticket: 7537 (new)
version_fixed: 1.10.4
status: resolved