blockchain: ensure all blocks get their longhash precalculated

If the number of blocks to check was not a multiple of the
number of preparation threads, the last few blocks would
not be included in the threaded long hash calculation.
Those would still get calculated when the block gets added
to the chain, however, so this was only a tiny performance
hit, rather than a security bug.

src/cryptonote_core/blockchain.cpp

@@ -3743,9 +3743,11 @@ bool Blockchain::prepare_handle_incoming_blocks(const std::list<block_complete_e
     if (!blocks_exist)
     {
       m_blocks_longhash_table.clear();
+      uint64_t thread_height = height;
       for (uint64_t i = 0; i < threads; i++)
       {
-        thread_list.push_back(new boost::thread(attrs, boost::bind(&Blockchain::block_longhash_worker, this, height + (i * batches), std::cref(blocks[i]), std::ref(maps[i]))));
+        thread_list.push_back(new boost::thread(attrs, boost::bind(&Blockchain::block_longhash_worker, this, thread_height, std::cref(blocks[i]), std::ref(maps[i]))));
+        thread_height += blocks[i].size();
       }
 
       for (size_t j = 0; j < thread_list.size(); j++)

src/cryptonote_core/blockchain.h

@@ -846,7 +846,7 @@ namespace cryptonote
      * @param blocks the blocks to be hashed
      * @param map return-by-reference the hashes for each block
      */
-    void block_longhash_worker(const uint64_t height, const std::vector<block> &blocks,
+    void block_longhash_worker(uint64_t height, const std::vector<block> &blocks,
         std::unordered_map<crypto::hash, crypto::hash> &map) const;
 
     /**