Backport from trunk:

htdbm: Fix possible buffer overflow if dbm database has very
long values.
PR: 30586
Submitted by: Dan Poirier


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@814852 13f79535-47bb-0310-9956-ffa450edef68