blockchain: add missing overflow check for already generated coins

When reaching the tail emission phase, the amount of coins will
eventually go over MONEY_SUPPLY, overflowing 64 bits. There was
a check added to blockchain_storage, but this was not ported to
the blockchain DB version.

Reported by smooth.

src/cryptonote_core/blockchain.cpp

@@ -2713,7 +2713,11 @@ leave:
   // populate various metadata about the block to be stored alongside it.
   block_size = cumulative_block_size;
   cumulative_difficulty = current_diffic;
-  already_generated_coins = already_generated_coins + base_reward;
+  // In the "tail" state when the minimum subsidy (implemented in get_block_reward) is in effect, the number of
+  // coins will eventually exceed MONEY_SUPPLY and overflow a uint64. To prevent overflow, cap already_generated_coins
+  // at MONEY_SUPPLY. already_generated_coins is only used to compute the block subsidy and MONEY_SUPPLY yields a
+  // subsidy of 0 under the base formula and therefore the minimum subsidy >0 in the tail state.
+  already_generated_coins = base_reward < (MONEY_SUPPLY-already_generated_coins) ? already_generated_coins + base_reward : MONEY_SUPPLY;
   if(m_db->height())
     cumulative_difficulty += m_db->get_block_cumulative_difficulty(m_db->height() - 1);