ce-page-wireguard: Fix memory leaks

This fixes memory leaks inside peer_allowed_ips_to_str() and in how it's used.

I tried to keep the changes minimal, except for the actual function which has been completely rewritten to prevent leaks. I'm completely rewriting this but that's for another much larger MR, but in the meantime let's prevent leaks.

The leaks where in the usage of the returned values from g_strconcat() and peer_allowed_ips_to_str(), which where never freed.

Signed-off-by: Adrien Plazas aplazas@gnome.org