Add assert to MergeOperator::PartialMergeMulti to check # of operands.

Summary:
Add assert(operands_list.size() >= 2) in MergeOperator::PartialMergeMulti
to ensure it's only be called when we have at least two merge operands.

Test Plan: run merge_test and stringappend_test.

Reviewers: haobo, igor

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17169