Adding nil to multicast event causes access violation when invoking

[Stefan Glienke]

When adding nil to a multicast event it will be added to the internal list which causes an AV when invoking the event.

var
  e: Event<TNotifyEvent>;
begin
  e.Add(nil);
  e.Invoke(nil); // <- AV

[Stefan Glienke]

fixed #244

→ <<cset 75af06b03dbf>>