mod_proxy: follow up to r1601291.

Since deferred_write_pool is needed by the core_output_filter and is a subpool
of the connection, shutdown in a pre_cleanup of the connection's pool to avoid
a freed memory access (SEGV).

Reported By: takashi


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1601630 13f79535-47bb-0310-9956-ffa450edef68