mod_deflate: Improve error detection when decompressing request bodies

with trailing garbage: handle case where trailing bytes are in
the same bucket.

trunk: http://svn.apache.org/r1502772

Submitted by: rjung
Reviewed by: jim, humbedooh


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1523268 13f79535-47bb-0310-9956-ffa450edef68