[vm] Fix flaky crash in StackTraceMallocHookLengthTest.

runtime/vm/profiler.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/address_sanitizer.h"
 #include "platform/memory_sanitizer.h"
 #include "platform/utils.h"
 
 #include "vm/allocation.h"
 #include "vm/atomic.h"
@@ skipping 66 matching lines @@
   Profiler::InitAllocationSampleBuffer();
   // Zero counters.
   memset(&counters_, 0, sizeof(counters_));
   ThreadInterrupter::InitOnce();
   ThreadInterrupter::SetInterruptPeriod(FLAG_profile_period);
   ThreadInterrupter::Startup();
   initialized_ = true;
 }
 
 void Profiler::InitAllocationSampleBuffer() {
-  if (FLAG_profiler_native_memory &&
-      (Profiler::allocation_sample_buffer_ == NULL)) {
+  ASSERT(Profiler::allocation_sample_buffer_ == NULL);
+  if (FLAG_profiler_native_memory) {
     Profiler::allocation_sample_buffer_ = new AllocationSampleBuffer();
   }
 }
 
 void Profiler::Shutdown() {
   if (!FLAG_profiler) {
     return;
   }
   ASSERT(initialized_);
   ThreadInterrupter::Shutdown();
@@ skipping 1594 matching lines @@
 }
 
 ProcessedSampleBuffer::ProcessedSampleBuffer()
     : code_lookup_table_(new CodeLookupTable(Thread::Current())) {
   ASSERT(code_lookup_table_ != NULL);
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart