Assert that the base path isn't null.

To protect against accidental regressions of the validator logic that
ends up calling this.

Author: grokys

src/GitHub.App/ViewModels/RepositoryCloneViewModel.cs

@@ -171,6 +171,8 @@ IObservable<Unit> OnCloneRepository(object state)
 
         bool IsAlreadyRepoAtPath(string path)
         {
+            Debug.Assert(path != null);
+
             bool isAlreadyRepoAtPath = false;
 
             if (SelectedRepository != null)