Univention Corporate Server 4.4 erratum 257 (security)

This update addresses the following issues:
* There is floating point exception at caca/dither.c (function
  caca_dither_bitmap). (CVE-2018-20544)
* There is an illegal WRITE memory access at common-image.c (function
  load_image) for 4bpp data. (CVE-2018-20545)
* There is an illegal READ memory access at caca/dither.c (function
  get_rgba_default) for the default bpp case. (CVE-2018-20546)
* There is an illegal READ memory access at caca/dither.c (function
  get_rgba_default) for 24bpp data. (CVE-2018-20547)
* There is an illegal WRITE memory access at common-image.c (function
  load_image) for 1bpp data. (CVE-2018-20548)
* There is an illegal WRITE memory access at caca/file.c (function
  caca_file_read). (CVE-2018-20549)