reflect another top request change in varnish-cache

Ref:    d67993451bda7eeb7c0ea3c5528a8353f283a190

src/vmod_blobdigest.c

@@ -308,9 +308,9 @@ get_scope(const struct vrt_ctx * const restrict ctx,
 			return NULL;
 		}
 
-		assert(ctx->req->top);
+		assert(ctx->req->topreq);
 		priv = VRT_priv_top(ctx, (void *)h);
-		ws = ctx->req->top->ws;
+		ws = ctx->req->topreq->ws;
 		break;
 	}
 	default: