Fix uninitialized gprof directory name in prefork MPM.

PR:           24450
Submitted by: Chris Knight <Christopher.D.Knight@nasa.gov>
Reviewed by:  Jeff Trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@101714 13f79535-47bb-0310-9956-ffa450edef68