Initialize apr_thread_t pointer to NULL so that apr_os_thread_put()

allocates a new one.  This resolves a crash seen on WinXP, caused
by r1103595.

Bad revision found by: Gregg L Smith <gls gknw.net>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1142264 13f79535-47bb-0310-9956-ffa450edef68