intel_th: msu: Fix missing allocation failure check on a kstrndup

Commit 615c164d ("intel_th: msu: Introduce buffer interface") forgot
to add a NULL pointer check for the value returned from kstrdup(), which
will be troublesome if the allocation fails.

Fix that by adding the check.

Addresses-Coverity: ("Dereference null return")
Fixes: 615c164d ("intel_th: msu: Introduce buffer interface")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
[alexander.shishkin: amended the commit message]
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/lkml/20190726120421.9650-1-colin.king@canonical.com/
Link: https://lore.kernel.org/r/20191028070651.9770-4-alexander.shishkin@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/intel_th/msu.c

@@ -1848,6 +1848,9 @@ mode_store(struct device *dev, struct device_attribute *attr, const char *buf,
 		len = cp - buf;
 
 	mode = kstrndup(buf, len, GFP_KERNEL);
+	if (!mode)
+		return -ENOMEM;
+
 	i = match_string(msc_mode, ARRAY_SIZE(msc_mode), mode);
 	if (i >= 0)
 		goto found;