btrfs: assert RAID stripe-extent length is always greater than 0

When modifying a RAID stripe-extent, ASSERT() that the length of the new
RAID stripe-extent is always greater than 0.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: morbidrsa

fs/btrfs/raid-stripe-tree.c

@@ -28,6 +28,7 @@ static void btrfs_partially_delete_raid_extent(struct btrfs_trans_handle *trans,
 		.offset = newlen,
 	};
 
+	ASSERT(newlen > 0);
 	ASSERT(oldkey->type == BTRFS_RAID_STRIPE_KEY);
 
 	leaf = path->nodes[0];