[qt] Remove assignment in if statement

Summary:
This serves 2 purposes:
 1. It is much more readable
 2. It works around a discrepancy between clang-format 12.0.0 and 12.0.1

There is no change in behavior.

Test Plan:
  ninja check-bitcoin-qt

Send a tx to check the modals work as intended

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14591

src/qt/bitcoingui.cpp

@@ -1245,9 +1245,9 @@ void BitcoinGUI::message(const QString &title, QString message,
 
     if (style & CClientUIInterface::MODAL) {
         // Check for buttons, use OK as default, if none was supplied
-        QMessageBox::StandardButton buttons;
-        if (!(buttons = (QMessageBox::StandardButton)(
-                  style & CClientUIInterface::BTN_MASK))) {
+        auto buttons = static_cast<QMessageBox::StandardButton>(
+            style & CClientUIInterface::BTN_MASK);
+        if (buttons) {
             buttons = QMessageBox::Ok;
         }