Univention Corporate Server 4.2 erratum 609 (extended maintenance)

This update addresses the following issues:
* New maintained dependency of Asterisk shostscript.
* write stack buffer overflow due to missing buffer length formatter in
  fscanf call (CVE-2017-17480)
* Excessive Iteration in opj_t1_encode_cblks (CVE-2018-6616)
* Division-by-zero vulnerabilities in the functions pi_next_pcrl,
  pi_next_cprl, and pi_next_rpcl (CVE-2018-14423)
* null pointer dereference caused by null image components in imagetopnm
  (CVE-2018-18088)