Fix a crash with r742218 by calling apr_os_thread_put() properly.

(The first parameter to apr_os_thread_put() must be initialized, either to
NULL or to an existing apr_thread_t.)



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@743894 13f79535-47bb-0310-9956-ffa450edef68