md: raid5 crash during degradation

NULL pointer access causes crash in raid5 module.
Signed-off-by: NAdam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -3070,7 +3070,7 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
 			if (sh->sector + STRIPE_SECTORS <= rdev->recovery_offset)
 				set_bit(R5_Insync, &dev->flags);
 		}
-		if (test_bit(R5_WriteError, &dev->flags)) {
+		if (rdev && test_bit(R5_WriteError, &dev->flags)) {
 			clear_bit(R5_Insync, &dev->flags);
 			if (!test_bit(Faulty, &rdev->flags)) {
 				s->handle_bad_blocks = 1;
@@ -3078,7 +3078,7 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
 			} else
 				clear_bit(R5_WriteError, &dev->flags);
 		}
-		if (test_bit(R5_MadeGood, &dev->flags)) {
+		if (rdev && test_bit(R5_MadeGood, &dev->flags)) {
 			if (!test_bit(Faulty, &rdev->flags)) {
 				s->handle_bad_blocks = 1;
 				atomic_inc(&rdev->nr_pending);