tx_pool: do not divide by 0

In case of a 0 tx weight, we use a placeholder value to insert in the
fee-per-byte set. This is used for pruning and mining, and those txes
are pruned, so will not be too large, nor added to the block template
if mining, so this is safe.

CID 204465

src/cryptonote_core/tx_pool.cpp

@@ -259,7 +259,7 @@ namespace cryptonote
           m_blockchain.add_txpool_tx(id, blob, meta);
           if (!insert_key_images(tx, id, kept_by_block))
             return false;
-          m_txs_by_fee_and_receive_time.emplace(std::pair<double, std::time_t>(fee / (double)tx_weight, receive_time), id);
+          m_txs_by_fee_and_receive_time.emplace(std::pair<double, std::time_t>(fee / (double)(tx_weight ? tx_weight : 1), receive_time), id);
           lock.commit();
         }
         catch (const std::exception &e)
@@ -305,7 +305,7 @@ namespace cryptonote
         m_blockchain.add_txpool_tx(id, blob, meta);
         if (!insert_key_images(tx, id, kept_by_block))
           return false;
-        m_txs_by_fee_and_receive_time.emplace(std::pair<double, std::time_t>(fee / (double)tx_weight, receive_time), id);
+        m_txs_by_fee_and_receive_time.emplace(std::pair<double, std::time_t>(fee / (double)(tx_weight ? tx_weight : 1), receive_time), id);
         lock.commit();
       }
       catch (const std::exception &e)