Make AAE key estimation API handle illegal IndexN
Make AAE key estimation API handle illegal IndexN

This commit changes riak_kv_index_hashtree to return not_responsible
when provided an illegal IndexN in estimate_keys/2. Prior to this
commit, an invalid IndexN would cause the hashtree process to crash.

Authored-by: Kresten Krab Thorup <a1cf3473e3c36561c3d5689ead914848760a7c43@trifork.com>
Rebased-by:  Joseph Blomstedt <16a9a54ddf4259952e3c118c763138e83693d7fd@basho.com>

andytill/riak_kv