qt, refactor: Replace if check with assert

There are no ways BitcoinGUI::updateWalletStatus being called without
an instance of the WalletFrame class.

src/qt/bitcoingui.cpp

@@ -1340,9 +1340,8 @@ void BitcoinGUI::setEncryptionStatus(int status)
 
 void BitcoinGUI::updateWalletStatus()
 {
-    if (!walletFrame) {
-        return;
-    }
+    assert(walletFrame);
+
     WalletView * const walletView = walletFrame->currentWalletView();
     if (!walletView) {
         return;