Merge branch 'ds/sparse-updates-oob-access-fix'

Merge branch 'ds/sparse-updates-oob-access-fix'

The code to skip unmerged paths in the index when sparse checkout
is in use would have made out-of-bound access of the in-core index
when the last path was unmerged, which has been corrected.

* ds/sparse-updates-oob-access-fix:
  unpack-trees: avoid array out-of-bounds error