mtd: check parts pointer before using it

The code has the check for parts but it called after kmemdup,
kmemdup(parts, sizeof(*parts) * nr_parts,...)
if (!parts)
	return -ENOMEM

In fact, we need check parts before safely using it.
and we also need check the real_parts to make sure kmemdup
allocation sucessfully.
Signed-off-by: Jason Liu <jason.hui@linaro.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>

drivers/mtd/mtdcore.c

@@ -465,12 +465,13 @@ int mtd_device_parse_register(struct mtd_info *mtd, const char **types,
 	struct mtd_partition *real_parts;
 
 	err = parse_mtd_partitions(mtd, types, &real_parts, parser_data);
-	if (err <= 0 && nr_parts) {
+	if (err <= 0 && nr_parts && parts) {
 		real_parts = kmemdup(parts, sizeof(*parts) * nr_parts,
 				     GFP_KERNEL);
-		err = nr_parts;
-		if (!parts)
+		if (!real_parts)
 			err = -ENOMEM;
+		else
+			err = nr_parts;
 	}
 
 	if (err > 0) {