http: follow up to r1484852.

Don't check LimitRequestBody when there is no (more) body.

This fixes an uninitialized use of 'totalread' in ap_http_filter() when either
the remaining or requested number of body bytes is zero, leading to an invalid
computation of bytes received.

Reported by: Michael Kaufmann <mail michael-kaufmann.ch>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1688274 13f79535-47bb-0310-9956-ffa450edef68