(sctp) Relax assertion in sun.nio.ch.sctp.AssociationChange::association

The assertion in sun.nio.ch.sctp.AssociationChange::association is too strong. It should be relaxed to allow a null association when the change event is CANT_START.

This can be verified by reading the specification from the supertype's, com.sun.nio.sctp.AssociationChangeNotification, association method, which says: "The association whose state has changed, or null if there is no association, that is CANT_START".

Existing tests already trip into this issue, when run with system assertions enabled.