models/component: fixed wrong port default-mapping

The default mapping which was inserted into port_mappings not meet
models.composition.rb:691:

	child_task.forward_ports(self_task, [child.port_mappings[port.actual_name], export_name] => Hash.new)

This results in some szenarios in a nil entry for the portmapping, which causes
the model-instanciation to fail.

This commit make the resolution of port consistend.