Improve safety of KDC AS-REQ processing cleanup

Improve safety of KDC AS-REQ processing cleanup

In finish_process_as_req(), don't clean up
state->reply.enc_part.ciphertext until the end of the function.
Otherwise a dangling pointer appears in the structure while it is used
several times, including potentially by audit plugin modules.

Omit the memset() to 0 for this and
state->ticket_reply.enc_part.ciphertext, as ciphertexts are not
secrets.