Fix Cursor.Last() on empty buckets.

@tv42 reported that creating a cursor on an empty bucket and then calling
Cursor.Last() causes an index out of range error and panics. This commit
adds a check for the page's item count being greater than zero.

Fixes #63.
Merge pull request #64 from benbjohnson/fix-empty-cursor-last

Fix Cursor.Last() on empty buckets.