mtd: physmap_of: fix potential NULL dereference

On device remove, when testing the cmtd field of an of_flash
struct to decide whether it is a concatenated device or not,
we get a false positive on cmtd == NULL, and dereference it
subsequently. This may occur if of_flash_remove() is called
from the cleanup path of of_flash_probe().

Instead, test for NULL first, and only then perform the test
for a concatenated device.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/maps/physmap_of.c

@@ -47,14 +47,12 @@ static int of_flash_remove(struct platform_device *dev)
 		return 0;
 	dev_set_drvdata(&dev->dev, NULL);
 
-	if (info->cmtd != info->list[0].mtd) {
+	if (info->cmtd) {
 		mtd_device_unregister(info->cmtd);
-		mtd_concat_destroy(info->cmtd);
+		if (info->cmtd != info->list[0].mtd)
+			mtd_concat_destroy(info->cmtd);
 	}
 
-	if (info->cmtd)
-		mtd_device_unregister(info->cmtd);
-
 	for (i = 0; i < info->list_size; i++) {
 		if (info->list[i].mtd)
 			map_destroy(info->list[i].mtd);