SI-8638  Empty UnrolledBuffer hangs on prepend.

Tracked down bug to incorrect recursion in insertAll.  Fixed by adding a missing case (which incidentally will provide better performance when adding to the end of a block).

No specific tests, as this is caught by quasi-complete collections tests.
Merge pull request #3840 from Ichoran/issue/8638

SI-8638  Empty UnrolledBuffer hangs on prepend.