s3: fix index out of range error with --fast-list fixes #2388

backend/s3/s3.go

@@ -864,7 +864,7 @@ func (f *Fs) list(dir string, recurse bool, fn listFn) error {
 			remote := key[rootLength:]
 			// is this a directory marker?
 			if (strings.HasSuffix(remote, "/") || remote == "") && *object.Size == 0 {
-				if recurse {
+				if recurse && remote != "" {
 					// add a directory in if --fast-list since will have no prefixes
 					remote = remote[:len(remote)-1]
 					err = fn(remote, &s3.Object{Key: &remote}, true)