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: nodejs#25814

PR-URL: nodejs#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>

src/env.cc

@@ -208,8 +208,7 @@ Environment::Environment(IsolateData* isolate_data,
   if (tracing::AgentWriterHandle* writer = GetTracingAgentWriter()) {
     trace_state_observer_ = std::make_unique<TrackingTraceStateObserver>(this);
     TracingController* tracing_controller = writer->GetTracingController();
-    if (tracing_controller != nullptr)
-      tracing_controller->AddTraceStateObserver(trace_state_observer_.get());
+    tracing_controller->AddTraceStateObserver(trace_state_observer_.get());
   }
 
   destroy_async_id_list_.reserve(512);
@@ -272,8 +271,7 @@ Environment::~Environment() {
     tracing::AgentWriterHandle* writer = GetTracingAgentWriter();
     CHECK_NOT_NULL(writer);
     TracingController* tracing_controller = writer->GetTracingController();
-    if (tracing_controller != nullptr)
-      tracing_controller->RemoveTraceStateObserver(trace_state_observer_.get());
+    tracing_controller->RemoveTraceStateObserver(trace_state_observer_.get());
   }
 
   delete[] heap_statistics_buffer_;

src/node_platform.cc

@@ -459,6 +459,7 @@ double NodePlatform::CurrentClockTimeMillis() {
 }
 
 TracingController* NodePlatform::GetTracingController() {
+  CHECK_NOT_NULL(tracing_controller_);
   return tracing_controller_;
 }
 

src/tracing/agent.h

@@ -81,7 +81,9 @@ class Agent {
   ~Agent();
 
   TracingController* GetTracingController() {
-    return tracing_controller_.get();
+    TracingController* controller = tracing_controller_.get();
+    CHECK_NOT_NULL(controller);
+    return controller;
   }
 
   enum UseDefaultCategoryMode {