Fix unhandled null pointer returning from os_addrinfo_intern

[ergl]

os_socket_listen, os_socket_connect and os_connect incorrectly
assumed that the return value from os_addrinfo_intern would never be
NULL, and therefore would call freeadrinfo with a null pointer, causing
a segmentation fault.

This should fix #3685.

[ergl]

I'll try to add a test case for this day, so this shouldn't be merged yet.

[ponylang-main]

Hi @ergl,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3687.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

@ergl still working on a test case?

[ergl]

Hi @SeanTAllen, I gave this a look, but I wasn't able to trigger a segfault like it happened in #3685. Even forcing a failure in getaddrinfo with LD_PRELOAD didn't cause a segfault.

In any case, I think it would be hard to have a reliable test for this, so maybe it's not needed. I'll write the release notes and push that later.

[SeanTAllen]

Sounds good @ergl.