clang-tidy: Fix modernize-use-default-member-init in headers

See https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-default-member-init.html

src/wallet/bdb.cpp

@@ -308,7 +308,7 @@ BerkeleyDatabase::~BerkeleyDatabase()
     }
 }
 
-BerkeleyBatch::BerkeleyBatch(BerkeleyDatabase& database, const bool read_only, bool fFlushOnCloseIn) : pdb(nullptr), activeTxn(nullptr), m_database(database)
+BerkeleyBatch::BerkeleyBatch(BerkeleyDatabase& database, const bool read_only, bool fFlushOnCloseIn) : m_database(database)
 {
     database.AddRef();
     database.Open();

src/wallet/bdb.h

@@ -207,9 +207,9 @@ private:
     bool HasKey(DataStream&& key) override;
 
 protected:
-    Db* pdb;
+    Db* pdb{nullptr};
     std::string strFile;
-    DbTxn* activeTxn;
+    DbTxn* activeTxn{nullptr};
     bool fReadOnly;
     bool fFlushOnClose;
     BerkeleyEnvironment *env;

src/wallet/db.h

@@ -123,7 +123,7 @@ class WalletDatabase
 {
 public:
     /** Create dummy DB handle */
-    WalletDatabase() : nUpdateCounter(0), nLastSeen(0), nLastFlushed(0), nLastWalletUpdate(0) {}
+    WalletDatabase() : nUpdateCounter(0) {}
     virtual ~WalletDatabase() {};
 
     /** Open the database if it is not already opened. */
@@ -165,9 +165,9 @@ public:
     virtual std::string Format() = 0;
 
     std::atomic<unsigned int> nUpdateCounter;
-    unsigned int nLastSeen;
-    unsigned int nLastFlushed;
-    int64_t nLastWalletUpdate;
+    unsigned int nLastSeen{0};
+    unsigned int nLastFlushed{0};
+    int64_t nLastWalletUpdate{0};
 
     /** Make a DatabaseBatch connected to this database */
     virtual std::unique_ptr<DatabaseBatch> MakeBatch(bool flush_on_close = true) = 0;

src/wallet/wallet.h

@@ -954,10 +954,10 @@ private:
     using Clock = std::chrono::steady_clock;
     using NowFn = std::function<Clock::time_point()>;
     CWallet& m_wallet;
-    bool m_could_reserve;
+    bool m_could_reserve{false};
     NowFn m_now;
 public:
-    explicit WalletRescanReserver(CWallet& w) : m_wallet(w), m_could_reserve(false) {}
+    explicit WalletRescanReserver(CWallet& w) : m_wallet(w) {}
 
     bool reserve()
     {