virtio-blk: fix NULL checking in virtblk_alloc_req()

Smatch complains about the inconsistent NULL checking here.  Fix it to
return NULL on failure.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (fixed accidental deletion)
diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 2edfb5c..53b81d5 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -90,10 +90,12 @@
 	struct virtblk_req *vbr;
 
 	vbr = mempool_alloc(vblk->pool, gfp_mask);
-	if (vbr && use_bio)
-		sg_init_table(vbr->sg, vblk->sg_elems);
+	if (!vbr)
+		return NULL;
 
 	vbr->vblk = vblk;
+	if (use_bio)
+		sg_init_table(vbr->sg, vblk->sg_elems);
 
 	return vbr;
 }