mpm_event: follow up to r1818804.

Address corner case where connection is aborted due to ap_run_pre_connection()
failure, and update comment about ap_run_process_connection() expected return
status and state.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1818951 13f79535-47bb-0310-9956-ffa450edef68