godoc/vfs: close file on error

When Stat() or IsDir() return errors, close the file to avoid a leak.

Change-Id: I46d5e34c3443413ca674f2a703d033d945c21efa
Reviewed-on: https://go-review.googlesource.com/10557
Reviewed-by: Minux Ma <minux@golang.org>

godoc/vfs/os.go

@@ -42,9 +42,11 @@ func (root osFS) Open(path string) (ReadSeekCloser, error) {
 	}
 	fi, err := f.Stat()
 	if err != nil {
+		f.Close()
 		return nil, err
 	}
 	if fi.IsDir() {
+		f.Close()
 		return nil, fmt.Errorf("Open: %s is a directory", path)
 	}
 	return f, nil