drivers/block/swim3.c: fix null pointer dereference

The use of pointer fs should be after the null check.
Signed-off-by: NCong Ding <dinggnu@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

drivers/block/swim3.c

@@ -1090,10 +1090,13 @@ static const struct block_device_operations floppy_fops = {
 static void swim3_mb_event(struct macio_dev* mdev, int mb_state)
 {
 	struct floppy_state *fs = macio_get_drvdata(mdev);
-	struct swim3 __iomem *sw = fs->swim3;
+	struct swim3 __iomem *sw;
 
 	if (!fs)
 		return;
+
+	sw = fs->swim3;
+
 	if (mb_state != MB_FD)
 		return;