Fix ksu crash in cases where it obtains the TGT

In order to allow ksu to use any locally-present service key for
verifying creds, the previous change to ksu switched from using a
retrieved or obtained TGT to fetch creds for the local "host" service,
and then passing those creds to krb5_verify_init_creds(), to passing the
retrieved TGT directly to krb5_verify_init_creds().  It did not take
care to retrieve the TGT from the temporary ccache if it had obtained
them, and in those cases it would attempt to verify NULL creds.