OpenSL ES: Fix buffer corruption

When start is called, the buffer is always filled starting from index 0
without regard to m_nexBuffer. m_nextBuffer could be set to 1 from the
previous playback which causes the second buffer to be filled first, which
is now the currently playing buffer. This is causing an audible hiccup
right after starting in cases where m_nextBuffer starts at 1.

Change-Id: Ia0d73638350d5258a51943d7a1c7cd6f22d068ee
Reviewed-by: Christian Stromme <christian.stromme@qt.io>

src/plugins/opensles/qopenslesaudiooutput.cpp

@@ -115,6 +115,7 @@ void QOpenSLESAudioOutput::start(QIODevice *device)
 
     m_pullMode = true;
     m_audioSource = device;
+    m_nextBuffer = 0;
     setState(QAudio::ActiveState);
     setError(QAudio::NoError);