don't try to close the temp file twice, otherwise Close() returns an error

```
--- FAIL: TestFile (0.00 seconds)
	file_test.go:171: /Users/rick/p/go-contentaddressable/file_test.go:37
		Expected: <nil>
		Actual:   invalid argument
```

The invalid argument came from trying to close the `w.tempFile` twice.  This sets it to nil so an extra `w.Close()` doesn't return an error.