src: fix nullptr dereference for signal during startup

This fixes a test failure when running
`test/parallel/test-child-process-spawnsync-kill-signal.js`
under load. What would happen is that `SignalExit()` tries
to shutdown the tracing agent, which might not have been set
up by the point that Node.js receives the signal.

PR-URL: https://github.com/nodejs/node/pull/20637


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>