If the sample hasn't been loaded, don't try and delete things.

Causes entirely reasonable warnings from QCoreApplication::postEvents,
when calling deleteLater on a null pointer.

Change-Id: I5f1fa526b2d57844b9c0d2e634d434e2f7867bdf
Reviewed-by: Ling Hu <ling.hu@nokia.com>

src/multimedia/effects/qsamplecache_p.cpp

@@ -282,8 +282,11 @@ void QSample::release()
 // must be called locked.
 void QSample::cleanup()
 {
-    m_waveDecoder->deleteLater();
-    m_stream->deleteLater();
+    if (m_waveDecoder)
+        m_waveDecoder->deleteLater();
+    if (m_stream)
+        m_stream->deleteLater();
+
     m_waveDecoder = 0;
     m_stream = 0;
 }