cmake-gui: Fix use-after-free in Open-possible check

Fix `QCMake::checkOpenPossible` to copy `toLocal8Bit().data()` before
its lifetime expires.

Source/QtDialog/QCMake.cxx

@@ -486,7 +486,7 @@ void QCMake::setWarnUnusedMode(bool value)
 
 void QCMake::checkOpenPossible()
 {
-  auto data = this->BinaryDirectory.toLocal8Bit().data();
+  std::string data = this->BinaryDirectory.toLocal8Bit().data();
   auto possible = this->CMakeInstance->Open(data, true);
   emit openPossible(possible);
 }