saferead must stop reading if the buffer has been marked in error

(i.e., a timeout has occurred).  Necessary for proxy timeouts.

Submitted by: Petr Lampa
Reviewed by: Roy Fielding


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@78183 13f79535-47bb-0310-9956-ffa450edef68