move a loop-invariant computation out of the thread loop and

at the same time quiet a gcc warning (it assumed that thread_limit
could be zero, which would leave ps_record uninitialized when
referenced after the end of the thread loop)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94579 13f79535-47bb-0310-9956-ffa450edef68