tls: fix legacy SecurePair clienthello race window

There is a time window between the first and the last step of processing
the clienthello event and the SecurePair may have been destroyed during
that interval.

Fixes: https://github.com/nodejs/node/issues/26428

PR-URL: https://github.com/nodejs/node/pull/26452


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>