Add null pointer check

The detailsText widget might be nullptr even if detailsButton is not.
Both are tested elsewhere in code, e.g. in the _q_buttonClicked
private slot, so be consistent.

Address static analyzer warning 00b85157447772462c3b8ffc1da0ae3a

Change-Id: Ib8c31b79653a6e8a9011692cb84ffaa2fb8047e7
Reviewed-by: Oliver Eftevaag <oliver.eftevaag@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/dialogs/qmessagebox.cpp

@@ -2043,7 +2043,7 @@ int QMessageBoxPrivate::showOldMessageBox(QWidget *parent, QMessageBox::Icon ico
 void QMessageBoxPrivate::retranslateStrings()
 {
 #if QT_CONFIG(textedit)
-    if (detailsButton)
+    if (detailsButton && detailsText)
         detailsButton->setLabel(detailsText->isHidden() ? ShowLabel : HideLabel);
 #endif
 }