Prevent "Uncaught TypeError: a(...).find(...)[0] is undefined"

It seems there is a rare race condition when creating the ical-links in the calendar table overview. at least sometimes no links are available while the table row already exists.

This is hard to reproduce; but the consequence is that some other document:ready events might not be executed (at least I have seen some panels not getting loaded once) and the fix to prevent the effect is rather easy, so I will just add that extra check without investigation why this actually happens (as IMO it should not be possible, but ...).