memory-infra: Avoid dereferencing polling_state_ when it could be null

base/trace_event/memory_dump_scheduler.cc

Index: base/trace_event/memory_dump_scheduler.cc
diff --git a/base/trace_event/memory_dump_scheduler.cc b/base/trace_event/memory_dump_scheduler.cc
index 66ea6c9f1aeb5dd98b8af1ac43d11a26b0d025be..150feb8e791759681585c17318b05cc52ed639e7 100644
--- a/base/trace_event/memory_dump_scheduler.cc
+++ b/base/trace_event/memory_dump_scheduler.cc
@@ -171,9 +171,11 @@ void MemoryDumpScheduler::RequestPeriodicGlobalDump() {
 }
 
 void MemoryDumpScheduler::PollMemoryOnPollingThread() {
-  if (polling_state_->current_state != PollingTriggerState::ENABLED)
+  if (!polling_state_)
     return;
 
+  DCHECK_EQ(PollingTriggerState::ENABLED, polling_state_->current_state);
+
   uint64_t polled_memory = 0;
   bool res = mdm_->PollFastMemoryTotal(&polled_memory);
   DCHECK(res);