Fix the way that ap_rwrite() checks for an error from ap_bwrite().

A dropped connection could cause a loop in ap_send_mmap() (and probably
a few other places) because of this problem.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85778 13f79535-47bb-0310-9956-ffa450edef68