src: nullcheck on trace controller

Insert a NULLCHECK prior to return. Ideally we do this in the caller, but the TraceController object is somewhat special as:

1. It is accessed by most threads
2. It's life cycle is managed by Agent::Agent
3. It's getter is invoked through Base Methods (upstream)

Refs: https://github.com/nodejs/node/issues/25814

cc @addaleax

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines