In core_input_filter, check for an empty brigade after

APR_BRIGADE_NORMALIZE().  Otherwise, we can get segfaults if a
client says it will post some data but we get FIN before any
data arrives.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93262 13f79535-47bb-0310-9956-ffa450edef68