Properly handle touch events with no touch points

Those can apparently happen on Android when putting your palm on the
screen, which in turn can cause crashes when accessing the out-of-bounds
first point.

ChangeLog

@@ -9,6 +9,7 @@ Unreleased Version 2.2.2-pre
  * Feature: Make size adjustment canvas shortcut depend on the speed of the drag. Thanks MorrowShore for suggesting.
  * Fix: Remove pointless permissions on Android that Qt includes by default but aren't actually used.
  * Fix: Don't default Android to fingerpaint when a pen is present.
+ * Fix: Work around a crash on Android that sometimes occurs when putting your palm on the screen, which somehow leads to touch events with zero contact points. Thanks Mav for reporting.
 
 2024-11-06 Version 2.2.2-beta.4
  * Fix: Solve rendering glitches with selection outlines that happen on some systems. Thanks xxxx for reporting.

src/desktop/utils/touchhandler.cpp

@@ -101,6 +101,11 @@ void TouchHandler::handleTouchBegin(QTouchEvent *event)
 {
 	const QList<compat::TouchPoint> &points = compat::touchPoints(*event);
 	int pointsCount = points.size();
+	// Can apparently happen on Android when putting your palm on the screen.
+	if(pointsCount == 0) {
+		return;
+	}
+
 	QPointF posf = compat::touchPos(points.first());
 
 	m_touchPos = QPointF(0.0, 0.0);
@@ -164,6 +169,11 @@ void TouchHandler::handleTouchUpdate(
 {
 	const QList<compat::TouchPoint> &points = compat::touchPoints(*event);
 	int pointsCount = points.size();
+	// Can apparently happen on Android when putting your palm on the screen.
+	if(pointsCount == 0) {
+		return;
+	}
+
 	if(pointsCount > m_maxTouchPoints) {
 		m_maxTouchPoints = pointsCount;
 	}
@@ -357,9 +367,10 @@ void TouchHandler::handleTouchEnd(QTouchEvent *event, bool cancel)
 			qUtf8Printable(compat::debug(points)),
 			qulonglong(event->timestamp()));
 		flushTouchDrawBuffer();
-		emit touchReleased(
-			QDateTime::currentMSecsSinceEpoch(),
-			compat::touchPos(compat::touchPoints(*event).first()));
+		// No points can happen on Android when putting your palm on the screen.
+		QPointF posf =
+			points.isEmpty() ? m_touchPos : compat::touchPos(points.first());
+		emit touchReleased(QDateTime::currentMSecsSinceEpoch(), posf);
 	} else if(m_touchDragging) {
 		DP_EVENT_LOG(
 			"touch_%s touching=%d type=%d device=%s points=%s timestamp=%llu",