http: Check for empty path and use "/"

Otherwise we end up with malloc(0) and all sorts of odd stuff happens

Change-Id: I5f3cac958d5d179dc5dbe08ad60928b59c7af5b5
Reviewed-on: http://review.couchbase.org/33105
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>