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.