model/action: bugfix - UNINIT actions do not have a Node

model/action: bugfix - UNINIT actions do not have a Node

Fix the one place where ModelChecker looks for a Node in an UNINIT
action.

This also adds an ASSERT() within ModelAction to prevent calling
get_node() on an UNINIT action. It is technically OK to call get_node()
in this case, but most callers don't check for NULL, so it's better to
just catch the ASSERT() for this unwanted special case.