ssl_util.c: pull up duplicated code in ssl_util_thr_id()

Should make it easier to see what's going on in the next few changes.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/trunk-openssl-threadid@1791730 13f79535-47bb-0310-9956-ffa450edef68