CBQE-0: minor fix in __merge_keys

Change-Id: I6bb609dcc89cee275fe1b7932730dbd4d54a1953
Reviewed-on: http://review.couchbase.org/40337
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>