Fix GSS context leak on error paths in process_gsstkey()

Fix GSS context leak on error paths in process_gsstkey()

After gss_accept_sec_context() succeeds, the GSS context is passed
to dst_key_fromgssapi() which transfers ownership to the dst_key.
If a subsequent operation fails (dst_key_fromgssapi itself,
dns_tsigkey_createfromkey, or dns_tsigkeyring_add), the cleanup
label frees the dst_key but only if it was created.  If the failure
happened before dst_key_fromgssapi, the GSS context was orphaned.

Delete the GSS context in the cleanup path when it was not
transferred to a dst_key.