ASoC: codecs: rt*-sdw: fix memory leak in set_sdw_stream()

Now that the sdw_stream is allocated in machine driver,
set_sdw_stream() is also called with a NULL argument during the
dailink shutdown.

In this case, the drivers should not allocate any memory, and just
return.

Detected with KASAN/kmemleak.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Reviewed-by: Rander Wang <rander.wang@intel.com>
Cc: Oder Chiou <oder_chiou@realtek.com>
Cc: Shuming Fan <shumingf@realtek.com>
Cc: Jack Yu <jack.yu@realtek.com>
Link: https://lore.kernel.org/r/20200515211531.11416-3-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/sound/soc/codecs/rt1308-sdw.c b/sound/soc/codecs/rt1308-sdw.c
index 4b88fa8..91cc5a15 100644
--- a/sound/soc/codecs/rt1308-sdw.c
+++ b/sound/soc/codecs/rt1308-sdw.c
@@ -482,6 +482,9 @@ static int rt1308_set_sdw_stream(struct snd_soc_dai *dai, void *sdw_stream,
 {
 	struct sdw_stream_data *stream;
 
+	if (!sdw_stream)
+		return 0;
+
 	stream = kzalloc(sizeof(*stream), GFP_KERNEL);
 	if (!stream)
 		return -ENOMEM;