Better assertions for worklist

Nodes on the worklist are never nexus nodes.

src/node.c

@@ -856,15 +856,14 @@ worklist_push(struct vdp_ctx *vdx, struct bytes_tree *tree,
 	AN(node);
 	while (node) {
 		parent = node->parent;
+		AN(parent);
 		next = VSTAILQ_NEXT(node, unpend);
 
-		if (parent != NULL) {
-			assert (parent->type == T_NEXUS);
-			if (parent->nexus.req->wrk == NULL)
-				parent->nexus.req->wrk = vdx->wrk;
-			else
-				assert (parent->nexus.req->wrk == vdx->wrk);
-		}
+		assert (parent->type == T_NEXUS);
+		if (parent->nexus.req->wrk == NULL)
+			parent->nexus.req->wrk = vdx->wrk;
+		else
+			assert (parent->nexus.req->wrk == vdx->wrk);
 
 		// not using assert_node: it needs locking
 		assert(node->state == ST_UNPENDING);