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

PR-URL: https://github.com/nodejs/node/pull/25943


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>