ab: Fix segfault in case the argument for -n is a very large number.

PR: 47178
Submitted by: Philipp Hagemeister <oss phihag.de>
Reviewed by: Stefan Fritsch


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@826497 13f79535-47bb-0310-9956-ffa450edef68