Make sure we actually stop the sync loop on logout

I think this was only a problem in the edgiest of edge conditions, but it
certainly didn't look right.

lib/sync.js

@@ -455,6 +455,7 @@ SyncApi.prototype._sync = function(syncOptions) {
             self._connectionReturnedDefer = null;
         }
         this._updateSyncState("STOPPED");
+        return;
     }
 
     var filterId = syncOptions.filterId;