xen/blkback: potential null dereference in error handling

blkbk->pending_pages can be NULL here so I added a check for it.
Signed-off-by: Dan Carpenter <error27@gmail.com>
[v1: Redid the loop a bit]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkback/blkback.c

@@ -809,11 +809,13 @@ static int __init xen_blkif_init(void)
  failed_init:
 	kfree(blkbk->pending_reqs);
 	kfree(blkbk->pending_grant_handles);
+	if (blkbk->pending_pages) {
 		for (i = 0; i < mmap_pages; i++) {
 			if (blkbk->pending_pages[i])
 				__free_page(blkbk->pending_pages[i]);
 		}
 		kfree(blkbk->pending_pages);
+	}
 	kfree(blkbk);
 	blkbk = NULL;
 	return rc;