ASoC: cache: Do the codec->reg_cache zero pionter check

For the snd_soc_cache_init(), the reg_size maybe zero and then the value
of codec->reg_cache, which is alloced via kzalloc, maybe equal to
ZERO_SIZE_PTR. If the reg parameter of snd_soc_cache_write() is large enough,
the cache[idx] = val maybe cause the kernel crash...

So this patch fix this via doing the zero pionter check of it.
Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

sound/soc/soc-cache.c

@@ -96,8 +96,7 @@ int snd_soc_cache_exit(struct snd_soc_codec *codec)
 {
 	dev_dbg(codec->dev, "ASoC: Destroying cache for %s codec\n",
 			codec->name);
-	if (!codec->reg_cache)
-		return 0;
+
 	kfree(codec->reg_cache);
 	codec->reg_cache = NULL;
 	return 0;
@@ -117,6 +116,7 @@ int snd_soc_cache_read(struct snd_soc_codec *codec,
 		return -EINVAL;
 
 	mutex_lock(&codec->cache_rw_mutex);
+	if (!ZERO_OR_NULL_PTR(codec->reg_cache))
 		*value = snd_soc_get_cache_val(codec->reg_cache, reg,
 					       codec->driver->reg_word_size);
 	mutex_unlock(&codec->cache_rw_mutex);
@@ -136,6 +136,7 @@ int snd_soc_cache_write(struct snd_soc_codec *codec,
 			unsigned int reg, unsigned int value)
 {
 	mutex_lock(&codec->cache_rw_mutex);
+	if (!ZERO_OR_NULL_PTR(codec->reg_cache))
 		snd_soc_set_cache_val(codec->reg_cache, reg, value,
 				      codec->driver->reg_word_size);
 	mutex_unlock(&codec->cache_rw_mutex);