fix assert_nexus for partially delivered / unpended subtrees

we do not unpend T_NEXUS nodes, so the following situation is perfectly
legal:

	T_NEXUS
	ST_CLOSED

T_NEXUS		T_DATA
ST_CLOSED	ST_UNPENDING

the purpose of the assertion is that, if a node below a nexus is
not yet unpended/delivered, the following need to be also, but this is
not true for T_NEXUS.

src/node_assert.h

@@ -61,6 +61,7 @@ assert_nexus(struct node *node, enum check_state check)
 	VSTAILQ_FOREACH(child, &node->nexus.children, sibling) {
 		assert_node(child, check);
 		if (check == CHK_ORDER &&
+		    child->type != T_NEXUS &&
 		    child->state <= ST_CLOSED)
 			check = CHK_PEND;
 	}