Fix #492: wait for async backup before destroying QETProject

writeBackup() fires QtConcurrent::run(QET::writeToFile, ..., &m_backup_file)
fire-and-forget: the QFuture was discarded and nothing kept m_backup_file
alive until the worker finished. If the QETProject was destroyed first, the
worker wrote through the freed member -> use-after-free crash in
QET::writeToFile (intermittent; ~1/6 on short-lived CLI runs).

Store the QFuture and waitForFinished() in ~QETProject (and before
setFilePath() re-points the managed backup file). Also skip launching a new
backup while one is still running, so two threads never write m_backup_file
at once.

The Qt6 path is still a TODO stub and the QtConcurrent block is KF5-only, so
this affects only the Qt5/KF5 build that actually has the backup code.

sources/qetproject.cpp

@@ -137,6 +137,11 @@ QETProject::QETProject(KAutoSaveFile *backup, QObject *parent) :
 */
 QETProject::~QETProject()
 {
+		//Wait for any in-flight async crash-recovery backup to finish: the worker
+		//writes through &m_backup_file, a member that would otherwise be destroyed
+		//under it (issue #492).
+	m_backup_future.waitForFinished();
+
 		//We block database signal to avoid hundreds of unnecessary emitted signal
 		//due to deletion (diagram, item, etc...) and as much update made in the not yet deleted things.
 	m_data_base.blockSignals(true);
@@ -339,6 +344,8 @@ void QETProject::setFilePath(const QString &filepath)
 	}
 #ifdef BUILD_WITHOUT_KF5
 #else
+		//Don't close/re-point the backup file while a backup is still writing it.
+	m_backup_future.waitForFinished();
 	if (m_backup_file.isOpen()) {
 		m_backup_file.close();
 	}
@@ -1809,8 +1816,12 @@ void QETProject::writeBackup()
 #ifdef BUILD_WITHOUT_KF5
 #else
 #	if QT_VERSION < QT_VERSION_CHECK(6, 0, 0) // ### Qt 6: remove
+		//Don't launch a new backup while the previous one is still writing:
+		//both would write through &m_backup_file on different threads.
+	if (m_backup_future.isRunning())
+		return;
 	QDomDocument xml_project(toXml());
-	QtConcurrent::run(
+	m_backup_future = QtConcurrent::run(
 				QET::writeToFile,xml_project,&m_backup_file,nullptr);
 #	else
 #		if TODO_LIST

sources/qetproject.h

@@ -35,6 +35,7 @@
 #endif
 
 #include <QHash>
+#include <QFuture>
 
 class Diagram;
 class ElementsLocation;
@@ -295,6 +296,7 @@ class QETProject : public QObject
 		bool m_freeze_new_conductors = false;
 		QTimer m_save_backup_timer,
 			   m_autosave_timer;
+		QFuture<bool> m_backup_future;
 #ifdef BUILD_WITHOUT_KF5
 #else
 		KAutoSaveFile m_backup_file;