ap_sub_req_output_filter: don't pass along a brigade if it becomes empty

after deleting the EOS bucket.

This prevents a seg fault in mod_include when the connection dies.  There
doesn't seem to be much point in passing empty brigades in general.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91225 13f79535-47bb-0310-9956-ffa450edef68