Univention Corporate Server 4.2 erratum 331 (security)

This update addresses the following issues:
* CVE-2017-2834 freerdp: Out-of-bounds write in license_recv()
* CVE-2017-2835 freerdp: Out-of-bounds write in rdp_recv_tpkt_pdu
* CVE-2017-2836 freerdp: Rdp Client Read Server Proprietary Certificate
  Denial of Service
* CVE-2017-2837 freerdp: Rdp Client GCC Read Server Security Data Denial of
  Service
* CVE-2017-2838 freerdp: Rdp Client License Read Product Info Denial of
  Service
* CVE-2017-2839 freerdp: Rdp Client License Read Challenge Packet Denial of
  Service