Fix 2 more unused reference errors VS2017

Summary:
As in #3425
Closes https://github.com/facebook/rocksdb/pull/3497

Differential Revision: D6979588

Pulled By: gfosco

fbshipit-source-id: e9fb32d04ad45575dfe9de1d79348d158e474197

db/memtable.cc

@@ -834,7 +834,8 @@ void MemTable::Update(SequenceNumber seq,
   }
 
   // key doesn't exist
-  bool add_res __attribute__((__unused__)) = Add(seq, kTypeValue, key, value);
+  bool add_res __attribute__((__unused__));
+  add_res = Add(seq, kTypeValue, key, value);
   // We already checked unused != seq above. In that case, Add should not fail.
   assert(add_res);
 }

db/write_batch.cc

@@ -1199,13 +1199,15 @@ class MemTableInserter : public WriteBatch::Handler {
                                                  value, &merged_value);
         if (status == UpdateStatus::UPDATED_INPLACE) {
           // prev_value is updated in-place with final value.
-          bool mem_res __attribute__((__unused__)) = mem->Add(
+          bool mem_res __attribute__((__unused__));
+          mem_res = mem->Add(
               sequence_, value_type, key, Slice(prev_buffer, prev_size));
           assert(mem_res);
           RecordTick(moptions->statistics, NUMBER_KEYS_WRITTEN);
         } else if (status == UpdateStatus::UPDATED) {
           // merged_value contains the final value.
-          bool mem_res __attribute__((__unused__)) =
+          bool mem_res __attribute__((__unused__));
+          mem_res =
               mem->Add(sequence_, value_type, key, Slice(merged_value));
           assert(mem_res);
           RecordTick(moptions->statistics, NUMBER_KEYS_WRITTEN);