Univention Corporate Server 4.2 erratum 346 (security)

This update addresses the following issues:
* The GIF decoding function gdImageCreateFromGifCtx does not zero colorMap
  arrays before use. A specially crafted GIF image could use the
  uninitialized tables to read ~700 bytes from the top of the stack,
  potentially disclosing sensitive information. (CVE-2017-7890)
* Double free vulnerability in the gdImagePngPtr function allows remote
  attackers to cause a denial of service via vectors related to a palette
  with no colors. (CVE-2017-6362)