gssd: Fix double free when exporting lucid context

When using GSSAPI's gss_krb5_export_lucid_context the context passed
into the function is actually deleted during the export (to avoid
reuse as the context contains state that depends on its usage).

Change the code to pass in a pointer to the context so that it can be
properly NULLed if we are using the GSSAPI context and following calls to
gss_delete_sec_context will not cause double free errors and segfaults.

Signed-off-by: Simo Sorce <simo@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>