ASoC: simple-card: Add cpu_dai and codec_dai names NULL check

The name of cpu DAI maybe omitted, and then strlen() will lead
kernel panic.
Signed-off-by: NXiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: NMark Brown <broonie@linaro.org>

sound/soc/generic/simple-card.c

@@ -142,6 +142,9 @@ static int asoc_simple_card_parse_of(struct device_node *node,
 	if (ret < 0)
 		return ret;
 
+	if (!info->cpu_dai.name || !info->codec_dai.name)
+		return -EINVAL;
+
 	/* card name is created from CPU/CODEC dai name */
 	name = devm_kzalloc(dev,
 			    strlen(info->cpu_dai.name)   +