AudioFileWave: test for properly opened output file

Do not pretend things gone right if output file could not be opened
for some reason.

Closes #539.

src/core/audio/AudioFileWave.cpp

@@ -2,7 +2,7 @@
  * AudioFileWave.cpp - audio-device which encodes wave-stream and writes it
  *                     into a WAVE-file. This is used for song-export.
  *
- * Copyright (c) 2004-2009 Tobias Doerffel <tobydox/at/users.sourceforge.net>
+ * Copyright (c) 2004-2013 Tobias Doerffel <tobydox/at/users.sourceforge.net>
  *
  * This file is part of Linux MultiMedia Studio - http://lmms.sourceforge.net
  *
@@ -38,9 +38,10 @@ AudioFileWave::AudioFileWave( const sample_rate_t _sample_rate,
 				mixer * _mixer ) :
 	AudioFileDevice( _sample_rate, _channels, _file, _use_vbr,
 			_nom_bitrate, _min_bitrate, _max_bitrate,
-								_depth, _mixer )
+								_depth, _mixer ),
+	m_sf( NULL )
 {
-	_success_ful = startEncoding();
+	_success_ful = outputFileOpened() && startEncoding();
 }
 
 
@@ -110,6 +111,9 @@ void AudioFileWave::writeBuffer( const surroundSampleFrame * _ab,
 
 void AudioFileWave::finishEncoding()
 {
-	sf_close( m_sf );
+	if( m_sf )
+	{
+		sf_close( m_sf );
+	}
 }