Crash less

Can't run any particles without a system set. Just skip the
'prepareFrame' function in that case.

Task-Number: QTBUG-20386
Change-Id: I22332636084d1696d5aff5386a5ca4ff0f2f1866
Reviewed-on: http://codereview.qt.nokia.com/1680


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>

src/declarative/particles/qsgitemparticle.cpp

@@ -165,6 +165,8 @@ QSGNode* QSGItemParticle::updatePaintNode(QSGNode* n, UpdatePaintNodeData* d)
 
 void QSGItemParticle::prepareNextFrame()
 {
+    if (!m_system)
+        return;
     qint64 timeStamp = m_system->systemSync(this);
     qreal curT = timeStamp/1000.0;
     qreal dt = curT - m_lastT;

src/declarative/particles/qsgmodelparticle.cpp

@@ -227,6 +227,8 @@ QSGNode* QSGModelParticle::updatePaintNode(QSGNode* n, UpdatePaintNodeData* d)
 
 void QSGModelParticle::prepareNextFrame()
 {
+    if (!m_system)
+        return;
     qint64 timeStamp = m_system->systemSync(this);
     qreal curT = timeStamp/1000.0;
     qreal dt = curT - m_lastT;