Fix KDC null dereference on large TGS replies

Fix KDC null dereference on large TGS replies

For TGS requests, dispatch() doesn't set state->active_realm, which
leads to a NULL dereference in finish_dispatch() if the reply is too
big for UDP.  Prior to commit 0a2f14f752c32a24200363cc6b6ae64a92f81379
the active realm was a global and was set when process_tgs_req()
called setup_server_realm().

Move TGS decoding out of process_tgs_req() so that we can set
state->active_realm before any errors requiring response.  Add a test
case.

[ghudson@mit.edu: edited commit message; added test case; reduced code
duplication; removed server handle from process_tgs_req() parameters]

ticket: 8666
tags: pullup
target_version: 1.16-next
target_version: 1.15-next