Complete the fix for bug 6100

Complete the fix for bug 6100

According to [MS-RPCE].pdf, section 2.2.2.11:

----
A client or a server that (during composing of a PDU) has allocated more space
for the authentication token than the security provider fills in SHOULD fill in
the rest of the allocated space with zero octets. These zero octets are still
considered to belong to the authentication token part of the PDU.<36>
----

RPC implementations are allowed to send padding bytes at the end of an auth
footer. Windows 7 makes use of this.

Thanks to Nick Meier <nmeier@microsoft.com>

Volker
(cherry picked from commit 7274d5691a339087f2770acf2f954830506f5cdc)