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]

ticket: 7952 (new)
target_version: 1.12.2
tags: pullup