net/http: handle absolute paths in mapDirOpenError

The current implementation does not account for Dir being
initialized with an absolute path on systems that start
paths with filepath.Separator. In this scenario, the
original error is returned, and not checked for file
segments.

This change adds a test for this case, and corrects the
behavior by ignoring blank path segments in the loop.

Refs #18984

Change-Id: I9b79fd0a73a46976c8e2feda0283ef0bb2b62ea1
Reviewed-on: https://go-review.googlesource.com/36804


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>