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: #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>

Author: addaleax

src/node.cc

@@ -326,7 +326,8 @@ static struct {
   }
 
   void StopTracingAgent() {
-    tracing_agent_->Stop();
+    if (tracing_agent_)
+      tracing_agent_->Stop();
   }
 
   tracing::Agent* GetTracingAgent() const {