LoadWallet: acquire cs_wallet mutex before clearing setKeyPool

Make the function mutex-aware, to prevent having to lock cs_wallet
at the call site in Init.

src/wallet.cpp

@@ -1489,7 +1489,6 @@ string CWallet::SendMoneyToDestination(const CTxDestination& address, int64_t nV
 
 DBErrors CWallet::LoadWallet(bool& fFirstRunRet)
 {
-    AssertLockHeld(cs_wallet); // setKeyPool
     if (!fFileBacked)
         return DB_LOAD_OK;
     fFirstRunRet = false;
@@ -1498,6 +1497,7 @@ DBErrors CWallet::LoadWallet(bool& fFirstRunRet)
     {
         if (CDB::Rewrite(strWalletFile, "\x04pool"))
         {
+            LOCK(cs_wallet);
             setKeyPool.clear();
             // Note: can't top-up keypool here, because wallet is locked.
             // User will be prompted to unlock wallet the next operation