CorePlugin: Don't call mutable expressions inside QTC_ASSERT

In order to conform to the theory: "Removing all
QTC_ASSERTs and QTC_CHECKs should not change anything".

Change-Id: I95593962486e1de1b980f17b8f924d6f98fa8e80
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/corejsextensions.cpp

@@ -132,7 +132,8 @@ QString UtilsJsExtension::mktemp(const QString &pattern) const
 
     QTemporaryFile file(tmp);
     file.setAutoRemove(false);
-    QTC_ASSERT(file.open(), return QString());
+    const bool isOpen = file.open();
+    QTC_ASSERT(isOpen, return {});
     file.close();
     return file.fileName();
 }