Treat LDAP KrbKey salt field as optional

Per the ASN.1 definition, the KrbKey salt field is optional.  Since
1.7, we have been treating it as mandatory in the encoder; since 1.11,
we have been treating it as mandatory in the decoder.  Mostly by luck,
we have been encoding a salt type of 0 when key_data_ver is 1, but we
really should not be looking at key_data_type[1] or key_data_length[1]
in this situation.  Treat the salt field as optional in the encoder
and decoder.  Although the previous commit ensures that we continue to
always encode a salt (without any dangerous assumptions about
krb5_key_data constructors), this change will allow us to decode key
data encoded by 1.6 without salt fields.

This also fixes issue #7918, by properly setting key_data_ver to 2 if
a salt type but no salt value is present.  It is difficult to get the
decoder to actually assign 2 to key_data_ver just because the salt
field is there, so take care of that in asn1_decode_sequence_of_keys.

ticket: 7919
target_version: 1.12.2
tags: pullup