Only cleanup new_bb, if it isn't NULL, fixing a crash in the event MPM when...

Only cleanup new_bb, if it isn't NULL, fixing a crash in the event MPM when write compeltion is active and a client disconnects.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@568202 13f79535-47bb-0310-9956-ffa450edef68