RFR: 8327370: (ch) sun.nio.ch.Poller.register throws AssertionError

[Daniel Fuchs]

Please find here an almost trivial change that fixes an intermittent AssertionError observed in `sun.nio.ch.Poller`.

This change (in the mist of a bigger fix) was contributed by @AlanBateman to the loom repo two weeks ago.

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Commit messages:
 - Fix mistake - catch Throwable to cover more cases
 - 8327370

Changes: https://git.openjdk.org/jdk/pull/19792/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19792&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327370
  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19792.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19792/head:pull/19792

PR: https://git.openjdk.org/jdk/pull/19792