gofmt formatter doesn't clear quickfix list when all errors are fixed

(Forwarding a report from someone else)

I am using the codefmt plugin for Go, by including:

``` vim
Glug codefmt gofmt_executable="goimports"
Glug codefmt-google
autocmd FileType go AutoFormatBuffer gofmt
```

And it looks like it doesn't clear the quick fix list when all errors are fixed. E.g.:
I have 3 errors, and if I save, the quick fix window shows the three of them.
If I fix 1, and save, the quick fix window correctly shows only two of them.
If now I fix the 2 remaining ones, and save, the quick fix window doesn't update at all.

It looks like we only call "setqflist" in the case of errors:
https://github.com/google/vim-codefmt/blob/master/autoload/codefmt.vim#L320
but never clear it if there are no errors.