Fix unlikely null dereference in TGS client code

Fix unlikely null dereference in TGS client code

If krb5_get_tgs_ktypes fails (due to an out-of-memory condition or an
error re-reading the profile), k5_make_tgs_req will dereference a null
pointer.  Check the return value before dereferencing defenctypes.

[ghudson@mit.edu: clarified commit message]

(cherry picked from commit 81e503c09ad9aef4a0afc6bf66a0269cde2151f3)

ticket: 7952
version_fixed: 1.12.2
status: resolved