os: make Readdir work as documented

Readdir's result should never contain a nil.

Fixes #5960.

R=golang-dev, rsc, bradfitz
CC=golang-dev
https://golang.org/cl/12261043
diff --git a/src/pkg/os/export_test.go b/src/pkg/os/export_test.go
index 9c6ef42..9fa7936 100644
--- a/src/pkg/os/export_test.go
+++ b/src/pkg/os/export_test.go
@@ -7,3 +7,4 @@
 // Export for testing.
 
 var Atime = atime
+var LstatP = &lstat