repo.workdir() crashes (SIGSEGV) on a bare repo

It should probably return null instead.

$ node gitrepo/badtest_git.js 
Created repo  /tmp/dummyrepo
Segmentation fault: 11

```
var git                 = require('nodegit');

git.Repo.init('/tmp/dummyrepo', true, function(err, repo) {
  if (err) throw err;
  console.log('Created repo');
  var workdir = repo.workdir();
  console.log('Got workdir', workdir);
});
```
