wallet2: fix background wallet detection in verify_password()

What changed, and why it matters

This one-line change fixes a logic bug in how Monero wallets detect 'background wallets' when verifying a password. The old code accidentally treated a JSON parsing failure as evidence that a wallet was a background wallet, and treated successful JSON parsing as evidence it was not. The fix reverses that logic. A background wallet is a special wallet type that lacks a private spend key. Because of the bug, a normal wallet with a valid JSON account record could be misclassified as a background wallet, which could lead the software to skip required spend-key checks or allow operations that should require the spend key. The actual security impact depends on how downstream code uses the no_spend_key flag, but the change clearly corrects an inverted condition that has security-relevant consequences.

Security signals we found

Evidence from the diff

Changed components

diff --git a/src/wallet/wallet2.cpp b/src/wallet/wallet2.cpp
index d7ecd39..d07a7c9 100644
--- a/src/wallet/wallet2.cpp
+++ b/src/wallet/wallet2.cpp
@@ -5444,7 +5444,7 @@ bool wallet2::verify_password(const std::string& keys_file_name, const epee::wip
     {
       get_custom_background_key(password, key, kdf_rounds);
       crypto::chacha20(keys_file_data.account_data.data(), keys_file_data.account_data.size(), key, keys_file_data.iv, &account_data[0]);
-      const bool is_background_wallet = json.Parse(account_data.c_str()).HasParseError() && json.IsObject();
+      const bool is_background_wallet = !json.Parse(account_data.c_str()).HasParseError() && json.IsObject();
       no_spend_key = no_spend_key || is_background_wallet;
     }
   }