Fix KDC encrypting key memory leak on some errors

Fix KDC encrypting key memory leak on some errors

Commit 0ba5ccd7bb3ea15e44a87f84ca6feed8890f657d separated the
allocation and destruction of encrypting_key, causing it to leak when
any of the intervening calls jump to the cleanup label.  Currently the
leak manifests on transited or authdata failures.  Move encrypting_key
destruction to the cleanup label so that it can't leak.  Reported by
anedvedicky@gmail.com.

(cherry picked from commit 1bcf2742d504a22b7354251bbc1e19c3dacd95f3)

ticket: 8645
version_fixed: 1.15.3