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

base/trace_event/memory_dump_scheduler.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/trace_event/memory_dump_scheduler.h"
 
 #include "base/process/process_metrics.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/trace_event/memory_dump_manager.h"
@@ skipping 153 matching lines @@
     level_of_detail = MemoryDumpLevelOfDetail::LIGHT;
   if (periodic_state_->heavy_dumps_rate > 0 &&
       periodic_state_->dump_count % periodic_state_->heavy_dumps_rate == 0)
     level_of_detail = MemoryDumpLevelOfDetail::DETAILED;
   ++periodic_state_->dump_count;
 
   mdm_->RequestGlobalDump(MemoryDumpType::PERIODIC_INTERVAL, level_of_detail);
 }
 
 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);
   if (polling_state_->level_of_detail == MemoryDumpLevelOfDetail::DETAILED) {
     TRACE_COUNTER1(MemoryDumpManager::kTraceCategory, "PolledMemoryMB",
                    polled_memory / 1024 / 1024);
   }
 
   if (ShouldTriggerDump(polled_memory)) {
     TRACE_EVENT_INSTANT1(MemoryDumpManager::kTraceCategory,
@@ skipping 130 matching lines @@
         last_memory_totals_kb[last_memory_totals_kb_index] * 1024;
   }
   num_polls_from_last_dump = 0;
   for (uint32_t i = 0; i < kMaxNumMemorySamples; ++i)
     last_memory_totals_kb[i] = 0;
   last_memory_totals_kb_index = 0;
 }
 
 }  // namespace trace_event
 }  // namespace base