Add AssertLockHeld to suppress clang error

clang showed the following error:
wallet/wallet.cpp:2577:17: error: calling function 'IsLockedCoin'
  requires holding mutex 'cs_wallet' exclusively
  [-Werror,-Wthread-safety-analysis]
            if (IsLockedCoin(entry.first, i)) {
                ^

Signed-off-by: Azat Nizametdinov <azat@thirdhash.com>

src/wallet/wallet.cpp

@@ -2562,6 +2562,9 @@ void CWallet::AvailableCoins(std::vector<COutput> &vCoins, bool fOnlySafe, const
 
         bool done = false;
         m_wallet_extension.ForEachMatureOutput(*pcoin->tx, block, nDepth, [&](const CTxOut &out, std::size_t i) {
+            AssertLockHeld(cs_main);
+            AssertLockHeld(cs_wallet);
+
             if (done) {
                 return;
             }