Fix NULL dereference in umem.c

Fix NULL dereference in umem.c

Signed-off-by: Neil Brown <[e-mail]>
Tested-by: Dave Chinner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/block/umem.c

···
 		page->idx++;
 		if (page->idx >= bio->bi_vcnt) {
 			page->bio = bio->bi_next;
-			page->idx = page->bio->bi_idx;
+			if (page->bio)
+				page->idx = page->bio->bi_idx;
 		}
 
 		pci_unmap_page(card->dev, desc->data_dma_handle,