Fix misplaced AssertLockHeld

Suggestion from MarcoFalke <falke.marco@gmail.com>
https://github.com/bitcoin/bitcoin/pull/17260#discussion_r340033021

src/wallet/wallet.cpp

@@ -249,8 +249,8 @@ const CWalletTx* CWallet::GetWalletTx(const uint256& hash) const
 
 void CWallet::UpgradeKeyMetadata()
 {
-    AssertLockHeld(m_spk_man->cs_wallet);
     if (m_spk_man) {
+        AssertLockHeld(m_spk_man->cs_wallet);
         m_spk_man->UpgradeKeyMetadata();
     }
 }