Fail if the new index is out of range. do not merge.

We were silently truncating, which made an obvious problem
into a non-obvious one.

Bug: http://code.google.com/p/android/issues/detail?id=40409

(cherry picked from commit 0a752f071fbbdfeab5dd9a230efe0f4f47b8cd94)

Change-Id: I6adbe6309bdcb2882f0e5500d6e1e149d08dbd2d