wallet: avoid returning a reference to vMasterKey after releasing the mutex that guards it

`CWallet::GetEncryptionKey()` would return a reference to the internal
`CWallet::vMasterKey`, guarded by `CWallet::cs_wallet`, which is unsafe.

Returning a copy would be a shorter solution, but could have security
implications of the master key remaining somewhere in the memory even
after `CWallet::Lock()` (the current calls to
`CWallet::GetEncryptionKey()` are safe, but that is not future proof).

So, instead of `EncryptSecret(m_storage.GetEncryptionKey(), ...)`
change the `GetEncryptionKey()` method to provide the encryption
key to a given callback:
`m_storage.WithEncryptionKey([](const CKeyingMaterial& k) { EncryptSecret(k, ...); })`

This silences the following (clang 18):

```
wallet/wallet.cpp:3520:12: error: returning variable 'vMasterKey' by reference requires holding mutex 'cs_wallet' [-Werror,-Wthread-safety-reference-return]
 3520 |     return vMasterKey;
      |            ^
```

src/wallet/scriptpubkeyman.cpp

@@ -811,7 +811,9 @@ bool LegacyScriptPubKeyMan::AddKeyPubKeyInner(const CKey& key, const CPubKey &pu
 
     std::vector<unsigned char> vchCryptedSecret;
     CKeyingMaterial vchSecret{UCharCast(key.begin()), UCharCast(key.end())};
-    if (!EncryptSecret(m_storage.GetEncryptionKey(), vchSecret, pubkey.GetHash(), vchCryptedSecret)) {
+    if (!m_storage.WithEncryptionKey([&](const CKeyingMaterial& encryption_key) {
+            return EncryptSecret(encryption_key, vchSecret, pubkey.GetHash(), vchCryptedSecret);
+        })) {
         return false;
     }
 
@@ -997,7 +999,9 @@ bool LegacyScriptPubKeyMan::GetKey(const CKeyID &address, CKey& keyOut) const
     {
         const CPubKey &vchPubKey = (*mi).second.first;
         const std::vector<unsigned char> &vchCryptedSecret = (*mi).second.second;
-        return DecryptKey(m_storage.GetEncryptionKey(), vchCryptedSecret, vchPubKey, keyOut);
+        return m_storage.WithEncryptionKey([&](const CKeyingMaterial& encryption_key) {
+            return DecryptKey(encryption_key, vchCryptedSecret, vchPubKey, keyOut);
+        });
     }
     return false;
 }
@@ -2128,7 +2132,9 @@ std::map<CKeyID, CKey> DescriptorScriptPubKeyMan::GetKeys() const
             const CPubKey& pubkey = key_pair.second.first;
             const std::vector<unsigned char>& crypted_secret = key_pair.second.second;
             CKey key;
-            DecryptKey(m_storage.GetEncryptionKey(), crypted_secret, pubkey, key);
+            m_storage.WithEncryptionKey([&](const CKeyingMaterial& encryption_key) {
+                return DecryptKey(encryption_key, crypted_secret, pubkey, key);
+            });
             keys[pubkey.GetID()] = key;
         }
         return keys;
@@ -2262,7 +2268,9 @@ bool DescriptorScriptPubKeyMan::AddDescriptorKeyWithDB(WalletBatch& batch, const
 
         std::vector<unsigned char> crypted_secret;
         CKeyingMaterial secret{UCharCast(key.begin()), UCharCast(key.end())};
-        if (!EncryptSecret(m_storage.GetEncryptionKey(), secret, pubkey.GetHash(), crypted_secret)) {
+        if (!m_storage.WithEncryptionKey([&](const CKeyingMaterial& encryption_key) {
+                return EncryptSecret(encryption_key, secret, pubkey.GetHash(), crypted_secret);
+            })) {
             return false;
         }
 

src/wallet/scriptpubkeyman.h

@@ -22,6 +22,7 @@
 
 #include <boost/signals2/signal.hpp>
 
+#include <functional>
 #include <optional>
 #include <unordered_map>
 
@@ -46,7 +47,8 @@ public:
     virtual void UnsetBlankWalletFlag(WalletBatch&) = 0;
     virtual bool CanSupportFeature(enum WalletFeature) const = 0;
     virtual void SetMinVersion(enum WalletFeature, WalletBatch* = nullptr) = 0;
-    virtual const CKeyingMaterial& GetEncryptionKey() const = 0;
+    //! Pass the encryption key to cb().
+    virtual bool WithEncryptionKey(std::function<bool (const CKeyingMaterial&)> cb) const = 0;
     virtual bool HasEncryptionKeys() const = 0;
     virtual bool IsLocked() const = 0;
 };

src/wallet/wallet.cpp

@@ -3513,9 +3513,10 @@ void CWallet::SetupLegacyScriptPubKeyMan()
     AddScriptPubKeyMan(id, std::move(spk_manager));
 }
 
-const CKeyingMaterial& CWallet::GetEncryptionKey() const
+bool CWallet::WithEncryptionKey(std::function<bool (const CKeyingMaterial&)> cb) const
 {
-    return vMasterKey;
+    LOCK(cs_wallet);
+    return cb(vMasterKey);
 }
 
 bool CWallet::HasEncryptionKeys() const

src/wallet/wallet.h

@@ -962,7 +962,8 @@ public:
     //! Make a LegacyScriptPubKeyMan and set it for all types, internal, and external.
     void SetupLegacyScriptPubKeyMan();
 
-    const CKeyingMaterial& GetEncryptionKey() const override;
+    bool WithEncryptionKey(std::function<bool (const CKeyingMaterial&)> cb) const override;
+
     bool HasEncryptionKeys() const override;
 
     /** Get last block processed height */