Univention Corporate Server 4.3 erratum 386 (security)

This update addresses the following issues:
* Memory safety bugs fixed (CVE-2018-12405)
* Buffer overflow and out-of-bounds read in ANGLE library with
  TextureStorage11 (CVE-2018-17466)
* Use-after-free with select element (CVE-2018-18492)
* Buffer overflow in accelerated 2D canvas with Skia (CVE-2018-18493)
* Same-origin policy violation using location attribute and
  performance.getEntries to steal cross-origin URLs (CVE-2018-18494)
* Integer overflow when calculating buffer sizes for images (CVE-2018-18498)