Free d in QAudioBuffer &QAudioBuffer::operator =(const QAudioBuffer &).

Task-number: QTBUG-40571
Change-Id: Idc2dcc7d254873edda653b9c9eb8d88cbb91d4f6
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>

src/multimedia/audio/qaudiobuffer.cpp

@@ -264,6 +264,8 @@ QAudioBuffer::QAudioBuffer(int numFrames, const QAudioFormat &format, qint64 sta
 QAudioBuffer &QAudioBuffer::operator =(const QAudioBuffer &other)
 {
     if (this->d != other.d) {
+        if (d)
+            d->deref();
         d = QAudioBufferPrivate::acquire(other.d);
     }
     return *this;