Fix several memory leaks in LDAP KDB modules

Fix memory leaks discovered by running valgrind over kdbtest, and some
related leaks.  Many of them result from not calling ldap_msgfree
after an unsuccessful search (as the OpenLDAP documentation requires)
or after an exception following a search, so many of the fixes move or
add ldap_msgfree calls to cleanup labels.

ldap_osa_free_princ_ent was not used, and could not be used because it
frees the container while krb5_lookup_tl_kadm_data uses a
caller-allocated container.  Change it to leave the container alone,
but to correctly destroy xdrs.  Use it in krb5_ldap_put_principal
where princ_ent was leaked.

In krb5_ldap_put_principal, subtreelist is declared twice in interior
scopes and not properly freed; move it to function scope and free it
up in the cleanup label.  Also in krb5_ldap_put_principal, avoiding
decoding multiple KBR5_TL_KADM_DATA values (which we don't expect to
see) as later decodes would cause earlier decodes to leak.

In krb5_encode_krbsecretkey, fix a leak of the krb5_data container and
also add an error check when calling asn1_encode_sequence_of_keys;
otherwise we would dereference a null pointer if we run out of memory
encoding keys (very unlikely).