RFR: 8371092: NullPointerException in AltServiceUsageTest.afterClass() test

[Daniel Fuchs]

On Sun, 2 Nov 2025 14:09:28 GMT, Peyang <duke at openjdk.org> wrote:

> - Add `safeClose(DatagramChannel)` to close udpNotResponding safely.
> - Null-check and exception handling unified with `safeStop()`.
> - Ensures test cleanup never fails due to uninitialized or problematic channel.

Please make sure to run the test locally before integrating. Networking tests are not run by github actions.

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Marked as reviewed by dfuchs (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28104#pullrequestreview-3410460082