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

db/merge_operator.cc

@@ -18,6 +18,7 @@ bool MergeOperator::PartialMergeMulti(const Slice& key,
                                       const std::deque<Slice>& operand_list,
                                       std::string* new_value,
                                       Logger* logger) const {
+  assert(operand_list.size() >= 2);
   // Simply loop through the operands
   std::string temp_value;
   Slice temp_slice(operand_list[0]);