DSA-6404-1 expat - security update

Multiple security issues were discovered in expat, an XML parsing C library,
including integer overflows, out-of-bounds write, NULL pointer dereferences,
excessive resource consumption, and memory corruption through re-entrant
parser API calls, which may result in denial of service or potentially the
execution of arbitrary code.

https://security-tracker.debian.org/tracker/DSA-6404-1