[GUI][Bug] Fix editing of CS address labels

Fixes a bug that prevented the editing of Cold Staking address labels in
the address book due to the passing of an empty purpose, which
`CWallet::ParseIntoAddress()` needs in order to determine if an address
is a cold staking type address.

Github-Pull: PIVX-Project#1588
Rebased-From: 805e436

src/wallet/wallet.cpp

@@ -2890,10 +2890,10 @@ bool CWallet::SetAddressBook(const CTxDestination& address, const std::string& s
             mapAddressBook[address].purpose = strPurpose;
     }
     NotifyAddressBookChanged(this, address, strName, ::IsMine(*this, address) != ISMINE_NO,
-        strPurpose, (fUpdated ? CT_UPDATED : CT_NEW));
+            mapAddressBook.at(address).purpose, (fUpdated ? CT_UPDATED : CT_NEW));
     if (!fFileBacked)
         return false;
-    std::string addressStr = ParseIntoAddress(address, strPurpose).ToString();
+    std::string addressStr = ParseIntoAddress(address, mapAddressBook.at(address).purpose).ToString();
     if (!strPurpose.empty() && !CWalletDB(strWalletFile).WritePurpose(addressStr, strPurpose))
         return false;
     return CWalletDB(strWalletFile).WriteName(addressStr, strName);