Univention Corporate Server 4.2 erratum 536 (extended maintenance)

This update addresses the following issues:
* heap-based buffer overflow in mspack/lzxd.c (CVE-2017-6419)
* Stack-based buffer over-read in cabd_read_string function (CVE-2017-11423)
* off-by-one error in the CHM PMGI/PMGL chunk number validity checks
  (CVE-2018-14679)
* off-by-one error in the CHM chunk number validity checks (CVE-2018-14680)
* Out-of-bounds Write in kwajd_read_headers in mspack/kwajd.c
  (CVE-2018-14681)
* off-by-one error in the TOLOWER() macro for CHM decompression
  (CVE-2018-14682)
* In mspack/cab.h in libmspack before 0.8alpha and cabextract before 1.8, the
  CAB block input buffer is one byte too small for the maximal Quantum block,
  leading to an out-of-bounds write. (CVE-2018-18584)
* chmd_read_headers in mspack/chmd.c in libmspack before 0.8alpha accepts a
  filename that has '\0' as its first or second character (such as the "/\0"
  name). (CVE-2018-18585)