godoc/zip.go: fix another zip file system bug
godoc/zip.go: fix another zip file system bug

Don't report that a directory was found just because we found
the list index where the directory would be if it were there...

R=iant
CC=golang-dev
http://codereview.appspot.com/4812051
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go