ENYO-575: Guard against invalid page index

Temporary fix for bogus page index problem that can cause invalid
elements to render in DataList. Permanent fix coming soon...

Enyo-DCO-1.1-Signed-Off-By: Gray Norton (gray.norton@lge.com)
Merge pull request #947 from enyojs/ENYO-575-quickfix-graynorton

ENYO-575: Guard against invalid page index

Reviewed-By: Aaron Tam (aaron.tam@lge.com) 
Integrated-By: Gray Norton (gray.norton@lge.com)