[krbdev.mit.edu #6647] Memory leak in kdc

[Greg Hudson via RT]

reply_encpart is treated as a container full of aliases, itself owning
nothing.

It would be easy enough to fix this memory leak by violating that
architecture (initialize reply_encpart.enc_padata earlier), free it in
the cleanup handler, but it would probably be better to preserve the
architecture.

return_enc_padata accepts a pointer to the entire reply_encpart but only
uses the enc_padata field.  We should modify it to take just the
enc_padata object and create a new automatic in process_as_req with cleanup.