do not call merge when checking to see if key exists

Summary:
Changes:
* added check for value before merge is called on code path that should check if key exists
Closes https://github.com/facebook/rocksdb/pull/2814

Reviewed By: IslamAbdelRahman

Differential Revision: D5743966

Pulled By: armishra

fbshipit-source-id: 6ac4283bc510c8ca50827d87ef0ba631f2b33b18

db/memtable.cc

@@ -593,10 +593,12 @@ static bool SaveValue(void* arg, const char* entry) {
         Slice v = GetLengthPrefixedSlice(key_ptr + key_length);
         *(s->status) = Status::OK();
         if (*(s->merge_in_progress)) {
+          if (s->value != nullptr) {
             *(s->status) = MergeHelper::TimedFullMerge(
                 merge_operator, s->key->user_key(), &v,
-              merge_context->GetOperands(), s->value, s->logger, s->statistics,
-              s->env_, nullptr /* result_operand */, true);
+                merge_context->GetOperands(), s->value, s->logger,
+                s->statistics, s->env_, nullptr /* result_operand */, true);
+          }
         } else if (s->value != nullptr) {
           s->value->assign(v.data(), v.size());
         }
@@ -610,10 +612,12 @@ static bool SaveValue(void* arg, const char* entry) {
       case kTypeSingleDeletion:
       case kTypeRangeDeletion: {
         if (*(s->merge_in_progress)) {
+          if (s->value != nullptr) {
             *(s->status) = MergeHelper::TimedFullMerge(
                 merge_operator, s->key->user_key(), nullptr,
-              merge_context->GetOperands(), s->value, s->logger, s->statistics,
-              s->env_, nullptr /* result_operand */, true);
+                merge_context->GetOperands(), s->value, s->logger,
+                s->statistics, s->env_, nullptr /* result_operand */, true);
+          }
         } else {
           *(s->status) = Status::NotFound();
         }