Extra protection on kdbldap_init

It might be work checking that the sess idx isn't already in use by
 calling getSession(idx) and returning a error is it doesn't return NULL.
 Alternatively, rather than having the user specifying the sess idx to
 kdbldap_init(), you could generate a unique value, e.g. use a session counter
 that gets incremented every time a session is added (but is not decremented
 on session remove)