mtd: mtdconcat: fix bug with uninitialized lock and unlock functions

Test if a lock or unlock function is present (pointer not NULL) before
calling it, to prevent a kernel dump.

Artem: removed extra blank lines
Signed-off-by: Martin Krause <martin.krause@tqs.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/mtdconcat.c

@@ -540,10 +540,12 @@ static int concat_lock(struct mtd_info *mtd, loff_t ofs, uint64_t len)
 		else
 			size = len;
 
+		if (subdev->lock) {
 			err = subdev->lock(subdev, ofs, size);
-
 			if (err)
 				break;
+		} else
+			err = -EOPNOTSUPP;
 
 		len -= size;
 		if (len == 0)
@@ -578,10 +580,12 @@ static int concat_unlock(struct mtd_info *mtd, loff_t ofs, uint64_t len)
 		else
 			size = len;
 
+		if (subdev->unlock) {
 			err = subdev->unlock(subdev, ofs, size);
-
 			if (err)
 				break;
+		} else
+			err = -EOPNOTSUPP;
 
 		len -= size;
 		if (len == 0)