krb5-gssapi: Remove several memory leaks.

Remove a leak seen on Kerberos/MIT (gss_OID is copied internally and
we were leaking it). Now we just pass NULL as advised in RFC2744.

|tmp| was never set back to buf->data.

Cleaned up Curl_sec_end to take into account failure in Curl_sec_login
(where conn->mech would be NULL but not conn->app_data or
conn->in_buffer->data).