pathpol UnmarshalJSON and UnmarshalYAML should check for default action

[fbuetler]

If one does unmarshal a json or yaml path policy like ["+ 42", "-"] it is not guaranteed that the last entry is going to be a default action.

It should have a similar check like the in NewACL:

// NewACL creates a new entry and checks for the presence of a default action
func NewACL(entries ...*ACLEntry) (*ACL, error) {
	if len(entries) == 0 || !entries[len(entries)-1].Rule.matchesAll() {
		return nil, ErrNoDefault
	}
	return &ACL{Entries: entries}, nil
}

Otherwise, using Eval on the malformed ACL is hitting this panic.