s3: krb5: keytab - The done label can be jumped to with context == NULL.

s3: krb5: keytab - The done label can be jumped to with context == NULL.

Ensure we don't crash in this case.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11959

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Uri Simchoni <uri@samba.org>
Autobuild-User(master): Uri Simchoni <uri@samba.org>
Autobuild-Date(master): Thu Jun  9 13:18:56 CEST 2016 on sn-devel-144

(cherry picked from commit e46cb9b835eb8f2bd998def82baf6f07fda9fe5c)

Autobuild-User(v4-2-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-2-test): Wed Jun 15 14:26:39 CEST 2016 on sn-devel-104