[bugfix] Avoid Nil Point Dereference on 404 (#717)

Netapp ONTAP S3 systems do not return a KeyCount when the requested object doesn't exist. This deserializes as nil and can cause a nil pointer dereference. Checking the pointer for nil first avoids this.

s3_fs.go

@@ -272,7 +272,7 @@ func (fs Fs) statDirectory(name string) (os.FileInfo, error) {
 			Err:  err,
 		}
 	}
-	if *out.KeyCount == 0 && name != "" {
+	if (out.KeyCount == nil || *out.KeyCount == 0) && name != "" {
 		return nil, &os.PathError{
 			Op:   "stat",
 			Path: name,