Univention Corporate Server 4.3 erratum 674 (extended maintenance)

This update addresses the following issues:
* Heap-based buffer overflow in the cpSeparateBufToContigBuf function
  resulting in a denial of service or possibly code execution
  (CVE-2018-12900)
* NULL pointer dereference in function _TIFFmemcmp at tif_unix.c
  (CVE-2018-17000)
* Integer overflow in multiply_ms in tools/ppm2tiff.c (CVE-2018-17100)
* NULL pointer dereference in TIFFWriteDirectorySec function in
  tif_dirwrite.c (CVE-2018-19210)
* integer overflow in libtiff/tif_dirwrite.c resulting in an invalid pointer
  dereference (CVE-2019-7663)
* integer overflow in _TIFFCheckMalloc and _TIFFCheckRealloc in tif_aux.c
  (CVE-2019-14973)
* integer overflow leading to heap-based buffer overflow in tif_getimage.c
  (CVE-2019-17546)