blkback: Fix potential resource leak.

drivers/xen/blkback/blkback.c

@@ -318,14 +318,14 @@ static int do_block_io_op(blkif_t *blkif)
 		if (RING_REQUEST_CONS_OVERFLOW(&blk_rings->common, rc))
 			break;
 
-		pending_req = alloc_req();
-		if (NULL == pending_req) {
-			blkif->st_oo_req++;
+		if (kthread_should_stop()) {
 			more_to_do = 1;
 			break;
 		}
 
-		if (kthread_should_stop()) {
+		pending_req = alloc_req();
+		if (NULL == pending_req) {
+			blkif->st_oo_req++;
 			more_to_do = 1;
 			break;
 		}