Zero out stack variables in process_tgs_req()

It is general good hygeine, preventing stack garbage from accidentally
leaking out into other consumers.

Don't bother with variables that are only used in a single place, but
do initialize the 'magic' field of nolrentry before it is used.
Remove unneeded variable enc_tkt_transited

There's no need to use an intermediate variable to initialize the
contents of enc_tkt_reply.transited.

Instead of setting each field to zero individually (and misspelling NULL),
use memset and set the one field which is being initialized to a nonzero
value explicitly.