Univention Corporate Server 4.2 erratum 539 (extended maintenance)

This update addresses the following issues:
* Integer overflow in multiply_ms in tools/ppm2tiff.c (CVE-2018-17100)
* Two out-of-bounds writes in cpTags in tools/tiff2bw.c and tools/pal2rgb.c
  (CVE-2018-17101)
* LibTIFF 4.0.9 (with JBIG enabled) decodes arbitrarily-sized JBIG into a
  buffer, ignoring the buffer size, which leads to a tif_jbig.c JBIGDecode
  out-of-bounds write. (CVE-2018-18557)