ab: Fix potential buffer overflows when processing the T and X
command-line options.

PR: 55360

Submitted by: Mike Rumph <mike.rumph oracle.com>
Reviewed by: trawick, jim, druggeri


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1515370 13f79535-47bb-0310-9956-ffa450edef68