Optimize Marionette.CollectionView..endBuffering

After upgrading from Marionette 1.1.0 to the latest version (1.6.2), the app broke down with the following error:

> Uncaught ItemViewContainerMissingError: The specified itemViewContainer was not found: tbody

(seems similar to #825).

In our case, the problem was solved after checking whether the buffer before continuing.