Further refinement for PR41056 / PR 19954 (mostly-fixed in r480135.)

We assume that a successful read but an empty brigade
is NOT cause for EAGAIN. Testing may or may not
confirm this assumption, in which case that test
may be required as well.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@504559 13f79535-47bb-0310-9956-ffa450edef68