Fix SEGFAULT when trying to connect to IPv6 peer in non-IPv6 environment

Fix SEGFAULT when trying to connect to IPv6 peer in non-IPv6 environment

Using my tinc setup I observe spurious SEGFAULTs in the daemon process.

My configuration comprises a proxy (type exec) and the peer's address is
given by its domain name. The domain resolves to both IPv4 and IPv6.
As IPv6 is not working in my environment, all connection attempts to the
resolved IPv6 addresses fail. Sometimes, after such a failure, the
segfault occurs.

Apparently, the issue is caused by a use after free due to failing to
reset a pointer.