tx_pool: catch theoretical error in get_block_reward

Coverity 196626

src/cryptonote_core/tx_pool.cpp

@@ -1268,7 +1268,11 @@ namespace cryptonote
     fee = 0;
     
     //baseline empty block
-    get_block_reward(median_weight, total_weight, already_generated_coins, best_coinbase, version);
+    if (!get_block_reward(median_weight, total_weight, already_generated_coins, best_coinbase, version))
+    {
+      MERROR("Failed to get block reward for empty block");
+      return false;
+    }
 
 
     size_t max_total_weight_pre_v5 = (130 * median_weight) / 100 - CRYPTONOTE_COINBASE_BLOB_RESERVED_SIZE;