hydrogen.cpp: change return value behavior of audioEngine_process()

Only return "2" if the consumer of audioEngine_process is the
DiskWriterDriver. Other driver may interprete the return value
in a different manner.

src/core/src/hydrogen.cpp

@@ -1269,10 +1269,15 @@ int audioEngine_process( uint32_t nframes, void* /*arg*/ )
 	 * The "try_lock" was introduced for Bug #164 (Deadlock after during
 	 * alsa driver shutdown). The try_lock *should* only fail in rare circumstances
 	 * (like shutting down drivers). In such cases, it seems to be ok to interrupt
-	 * audio processing.
+	 * audio processing. Returning the special return value "2" enables the disk 
+	 * writer driver to repeat the processing of the current data.
 	 */
-	if(!AudioEngine::get_instance()->try_lock( RIGHT_HERE )){
-		return 2;
+	if(!AudioEngine::get_instance()->try_lock( RIGHT_HERE )) {
+		if ( m_pAudioDriver->class_name() == DiskWriterDriver::class_name() ) {
+			return 2;	// inform the caller that we could not aquire the lock
+		}
+
+		return 0;
 	}
 
 	if ( m_audioEngineState < STATE_READY) {