Wait for sub requests to finish before deleting the node

Fixes https://gitlab.com/uplex/varnish/libvdp-pesi/-/issues/10

src/node.c

@@ -622,6 +622,8 @@ fini_subreq(const struct vdp_ctx *vdx, struct node *node)
 
 	assert(node->type == T_SUBREQ);
 
+	if (node->subreq.done == 0)
+		subreq_wait_done(node);
 	AN(node->subreq.done);
 
 	if (node->subreq.req == NULL) {