db: throw when given a non txout_to_key output to add

The check was explicit in the original version, so it seems
safer to make it explicit here, especially as it is now done
implicitely in a different place, away from the original check.

src/blockchain_db/berkeleydb/db_bdb.cpp

@@ -391,6 +391,10 @@ void BlockchainBDB::add_output(const crypto::hash& tx_hash, const tx_out& tx_out
         if (m_output_keys->put(DB_DEFAULT_TX, &k, &data, 0))
             throw0(DB_ERROR("Failed to add output pubkey to db transaction"));
     }
+    else
+    {
+      throw0(DB_ERROR("Wrong output type: expected txout_to_key"));
+    }
 
     m_num_outputs++;
 }

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -695,6 +695,10 @@ void BlockchainLMDB::add_output(const crypto::hash& tx_hash, const tx_out& tx_ou
     if (mdb_put(*m_write_txn, m_output_keys, &k, &data, 0))
       throw0(DB_ERROR("Failed to add output pubkey to db transaction"));
   }
+  else
+  {
+    throw0(DB_ERROR("Wrong output type: expected txout_to_key"));
+  }
 
   m_num_outputs++;
 }

src/cryptonote_core/blockchain.cpp

@@ -2246,6 +2246,11 @@ bool Blockchain::check_tx_input(const txin_to_key& txin, const crypto::hash& tx_
         return false;
       }
 
+      // The original code includes a check for the output corresponding to this input
+      // to be a txout_to_key. This is removed, as the database does not store this info,
+      // but only txout_to_key outputs are stored in the DB in the first place, done in
+      // Blockchain*::add_output
+
       m_output_keys.push_back(pubkey);
       return true;
     }