fix segv triggered by recent ap_lingering_close change

need to set SSLFilterRec.pssl = NULL when ssl_hook_CloseConnection is called
otherwise, ap_lingering_close -> ap_flush_conn will call ssl_io_filter_Output
which thinks it can still use the SSLFilterRec.pssl
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk/modules/ssl@91886 13f79535-47bb-0310-9956-ffa450edef68