vrt: Make sure VRT_Rollback() has a snapshot

Otherwise we would entirely unravel the task workspace.

bin/varnishd/cache/cache_fetch.c

@@ -153,6 +153,7 @@ Bereq_Rollback(VRT_CTX)
 	HTTP_Clone(bo->bereq, bo->bereq0);
 	bo->vfp_filter_list = NULL;
 	bo->err_reason = NULL;
+	AN(bo->ws_bo);
 	WS_Rollback(bo->ws, bo->ws_bo);
 }
 

bin/varnishd/cache/cache_req.c

@@ -209,6 +209,7 @@ Req_Rollback(VRT_CTX)
 	req->filter_list = NULL;
 	if (WS_Overflowed(req->ws))
 		req->wrk->stats->ws_client_overflow++;
+	AN(req->ws_req);
 	WS_Rollback(req->ws, req->ws_req);
 }