Don't fall thru to Hydra::View::NARInfo

This caused exceptions like:

  Caught exception in Hydra::Controller::Build->view_build "writing to file: Broken pipe at /nix/store/ihdb3widsq1dk7sbl5vqjxfcxb5ypad4-hydra-0.1pre1297-8158093/libexec/hydra/lib/Hydra/Controller/Build.pm line 59."

because the connection to the Nix daemon would be terminated due to a
protocol violation (calling queryPathInfo with an empty string).

src/lib/Hydra/Controller/Root.pm

@@ -225,8 +225,9 @@ sub narinfo :LocalRegex('^([a-z0-9]+).narinfo$') :Args(0) {
         $c->response->content_type('text/plain');
         $c->stash->{plain}->{data} = "does not exist\n";
         $c->forward('Hydra::View::Plain');
+        return;
     }
-    
+
     $c->stash->{storePath} = $path;
     $c->forward('Hydra::View::NARInfo');
 }