wallet: bugfix, invalid crypted key "checksum_valid" set

At wallet load time, we set the crypted key "checksum_valid" variable always to false.
Which, on every wallet decryption call, forces the process to re-write the entire ckeys to db when
it's not needed.

Author: furszy

src/wallet/walletdb.cpp

@@ -482,7 +482,7 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
             if (!ssValue.eof()) {
                 uint256 checksum;
                 ssValue >> checksum;
-                if ((checksum_valid = Hash(vchPrivKey) != checksum)) {
+                if (!(checksum_valid = Hash(vchPrivKey) == checksum)) {
                     strErr = "Error reading wallet database: Encrypted key corrupt";
                     return false;
                 }