Do not lose log messages with NULL server_rec when error log provider is used.

- set stderr_log to NULL after it is redirected to /dev/null
- use log provider of ap_server_conf in log_error_core when no server_rec
  is provided and std_err_log is NULL


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1539988 13f79535-47bb-0310-9956-ffa450edef68