blockchain: fix innocuous difficulty cache inconsistency

This inconsistent state would not actually be used in practice

src/cryptonote_core/blockchain.cpp

@@ -831,6 +831,7 @@ difficulty_type Blockchain::get_difficulty_for_next_block()
   std::vector<uint64_t> timestamps;
   std::vector<difficulty_type> difficulties;
   auto height = m_db->height();
+  top_hash = get_tail_id(); // get it again now that we have the lock
   // ND: Speedup
   // 1. Keep a list of the last 735 (or less) blocks that is used to compute difficulty,
   //    then when the next block difficulty is queried, push the latest height data and