prefork: Don't segfault when we are able to listen on some but

not all of the configured ports.

Other points to ponder:

  why no log message for the segfault?

  similar change would seem to be needed in other MPMs...  we
  may be adding bogus entries to the poll set


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91399 13f79535-47bb-0310-9956-ffa450edef68