agents: fix crash accessing cont prof queue

On runtime shutdown, when continuous profiling is enabled, it can happen
that we try access `GrpcAgent::cont_prof_queue_` when it has already
being reset, thus crashing the process.

PR-URL: #348
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>

Author: santigimeno

agents/grpc/src/grpc_agent.cc

@@ -814,7 +814,7 @@ int GrpcAgent::start_heap_snapshot_from_js(
     const ProfileCollector::ProfileQStor& profile_data,
     WeakGrpcAgent agent_wp) {
   SharedGrpcAgent agent = agent_wp.lock();
-  if (agent == nullptr) {
+  if (agent == nullptr || agent->cont_profile_queue_ == nullptr) {
     return;
   }