ALSA: hda: Avoid NULL pointer dereference at snd_hdac_stream_start()

For ca0132 codec, azx_dev->stream is NULL during firmware loading.
Calling snd_hdac_get_stream_stripe_ctl unconditionally causes NULL
pointer dereference in that function.

Fixes: 9b6f7e7 ("ALSA: hda: program stripe bits for controller")
Signed-off-by: Mariusz Ceier <mceier+kernel@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/hda/hdac_stream.c

@@ -95,7 +95,10 @@ void snd_hdac_stream_start(struct hdac_stream *azx_dev, bool fresh_start)
 			      1 << azx_dev->index,
 			      1 << azx_dev->index);
 	/* set stripe control */
-	stripe_ctl = snd_hdac_get_stream_stripe_ctl(bus, azx_dev->substream);
+	if (azx_dev->substream)
+		stripe_ctl = snd_hdac_get_stream_stripe_ctl(bus, azx_dev->substream);
+	else
+		stripe_ctl = 0;
 	snd_hdac_stream_updateb(azx_dev, SD_CTL_3B, SD_CTL_STRIPE_MASK,
 				stripe_ctl);
 	/* set DMA start and interrupt mask */