ASoC: SOF: topology: fix missing NULL pointer check

Add check to avoid possible NULL pointer dereference issue.

This issue was reported by static analysis tools, we didn't face this
issue but we can't rule it out either as a false positive.

Reported-by: Keqiao Zhang <keqiao.zhang@intel.com>
Signed-off-by: Keyon Jie <yang.jie@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191101170916.26517-2-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/topology.c

@@ -150,6 +150,11 @@ static int sof_keyword_dapm_event(struct snd_soc_dapm_widget *w,
 
 	/* get runtime PCM params using widget's stream name */
 	spcm = snd_sof_find_spcm_name(sdev, swidget->widget->sname);
+	if (!spcm) {
+		dev_err(sdev->dev, "error: cannot find PCM for %s\n",
+			swidget->widget->name);
+		return -EINVAL;
+	}
 
 	/* process events */
 	switch (event) {