Fixes #2042 

This is a typical TOCTOU (time-of-check/time-of-use) race
https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use.
The NPE was occurring because the channel field could be set to null by
another thread between the check and its use:

if (channel != null) {                 // time-of-check
    Channels.setDiscard(channel);      //  time-of-use
    Channels.silentlyCloseChannel(channel);
}

By copying channel into a local variable in one atomic read, we ensure
that—even if another thread changes the field—the local reference
remains valid.

P.S. It is hard to write a deterministic test that fails consistently,
so this PR only includes the code fix.

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Co-authored-by: prat <[email protected]>