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.comSigned-off-by: Mark Brown <broonie@kernel.org>