Fix: rapid/continuous keyboard Next event under Topics List view may lose selected item on "load more"

Using Next keyboard shortcut (J) rapidly while reaching last item in the topic list and
causing a "load more". The handler might get into a state where it can not detect any selected item due to the delay from Em.run.next
and will result in position reset.

+added boundary check of last item on next, early in the handler to avoid unnecessary computations 


https://meta.discourse.org/t/keyboard-shortcut-next-previous-continues-key-down-reset-to-first-item-in-list-on-load-more/20042
Merge pull request #2798 from lidlanca/patch-3

Fix: rapid/continuous keyboard Next event under Topics List view may lose selected item on "load more"