fix for #498 -- if a return header overran the obj_workspace (but was small enough to fit in sess_workspace) the header is dropped and an empty slot is put in there -- rest of the code now detects this empty slot. It should probably ask VLC what to do if a header is dropped

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4038 d4fa192b-c00b-0410-8231-f00ffab90ce4