The parent-provides model is destroyed if no <parent-provides> node is
found in the configuration, which resulted in
Warning: list model not empty at destruction time
and leaking memory for the allocated nodes. The commit now explicitly
empties the list model in the destructor of ~Parent_provides_model.
Note, the case is implicitly tested in pkg/test-init by step "denial of
forwarded session request" and <init_config version="empty">.
Thanks to Peter for reporting this issue.
Fixes#4547
