This change fixes multiple denial-of-service vulnerabilities:
 * CVE-2016-9131: A malformed response to an ANY query can cause an
   assertion failure during recursion
 * CVE-2016-9147: An error handling a query response containing
   inconsistent DNSSEC information could cause an assertion failure
 * CVE-2016-9444: An unusually-formed DS record response could cause
   an assertion failure
 * CVE-2016-9778: An error handling certain queries using the
   nxdomain-redirect feature could cause a REQUIRE assertion failure
   in db.c

Signed-off-by: Noah Meyerhans <frodo@morgul.net>