Fix misplaced sec_trailer in rpc_auth_3 PDU

This fixes the placement of the sec_trailer when sending an rpc_auth_3 PDU; it was shifted by 4 bytes.
This was caused by writing 24 bytes from the rpcconn_rpc_auth_3_hdr_t, although there are only 20 bytes defined in it before the auth_verifier element.
It did work because there is a strict requirement in [MS-RPCE] 2.2.2.11 on how to calculate the start of the sec_trailer. The resulting PDU structure still fulfilled that requirement, even though there was no other indication of those extra bytes in the PDU.