Changing number of events causes "Invalid argument supplied for foreach"

Steps to Reproduce:
- Create a calendar block with 5 items displayed
- Edit the calendar block to now display > 5 items

Expected Results:
- 10 items display instead of 5

Actual Results:
- PHP Error: Invalid argument supplied for foreach

Possible Cause:
- XML feeds are cached by SimplePie. SimplePie has no easy way of knowing that are now looking for >5 items instead of 5. It's possible that the old cache is being used and is trying to loop >5 times over only 5 items.

Solution:
- should fail more gracefully when not enough items are available.