ab: Fix crash with -W when exiting earlya on fatal error.

ab: Fix crash with -W when exiting earlya on fatal error.

When multiple threads are failing (e.g. read/write timeout) it's not
thread-safe to simply/concurrently print the stats and exit. This can
result in garbage being printed or a crash.

Let's cleanly shutdown the threads and do the printing at a single point.
For the places where we want to fail but threads are not started yet we
can simply exit still, so to simplify the stats are not printed in an
atexit() handler.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1910513 13f79535-47bb-0310-9956-ffa450edef68