ASoC: SOF: sof-audio: prepare_widgets: Check swidget for NULL on sink failure

If the swidget is NULL we skip the preparing of the widget and jump to
handle the sink path of the widget.
If the prepare fails in this case we would undo the prepare but the swidget
is NULL (we skipped the prepare for the widget).

To avoid NULL pointer dereference in this case we must check swidget
against NULL pointer once again.

Fixes: 0ad84b11 ("ASoC: SOF: sof-audio: skip prepare/unprepare if swidget is NULL")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230120102125.30653-1-peter.ujfalusi@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/sof-audio.c

@@ -327,7 +327,8 @@ sof_prepare_widgets_in_path(struct snd_sof_dev *sdev, struct snd_soc_dapm_widget
 			p->walking = false;
 			if (ret < 0) {
 				/* unprepare the source widget */
-				if (widget_ops[widget->id].ipc_unprepare && swidget->prepared) {
+				if (widget_ops[widget->id].ipc_unprepare &&
+				    swidget && swidget->prepared) {
 					widget_ops[widget->id].ipc_unprepare(swidget);
 					swidget->prepared = false;
 				}