Univention Corporate Server 4.3 erratum 182 (security)

This update addresses the following issues:
* Heap-based buffer over-read in the ReadSIXELImage function in
  coders/sixel.c (CVE-2018-5248)
* heap-based buffer over-read in ReadSUNImage in coders/sun.c
  (CVE-2018-11251)
* out of bounds write in ReadBMPImage and WriteBMPImage in coders/bmp.c
  (CVE-2018-12599)
* out of bounds write ReadDIBImage and WriteDIBImage in coders/dib.c
  (CVE-2018-12600)