refactor: SetAddressBookWithDB, minimize number of map lookups

Author: furszy

src/wallet/wallet.cpp

@@ -2359,14 +2359,15 @@ bool CWallet::SetAddressBookWithDB(WalletBatch& batch, const CTxDestination& add
     {
         LOCK(cs_wallet);
         std::map<CTxDestination, CAddressBookData>::iterator mi = m_address_book.find(address);
-        fUpdated = (mi != m_address_book.end() && !mi->second.IsChange());
-        m_address_book[address].SetLabel(strName);
+        fUpdated = mi != m_address_book.end() && !mi->second.IsChange();
+
+        CAddressBookData& record = mi != m_address_book.end() ? mi->second : m_address_book[address];
+        record.SetLabel(strName);
         is_mine = IsMine(address) != ISMINE_NO;
         if (new_purpose) { /* update purpose only if requested */
-            purpose = m_address_book[address].purpose = new_purpose;
-        } else {
-            purpose = m_address_book[address].purpose;
+            record.purpose = new_purpose;
         }
+        purpose = record.purpose;
     }
 
     const std::string& encoded_dest = EncodeDestination(address);