Fixes#2042

This is a typical TOCTOU (time-of-check/time-of-use) racehttps://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.