[vm] Fix flaky crash in StackTraceMallocHookLengthTest.

runtime/vm/profiler.cc

Index: runtime/vm/profiler.cc
diff --git a/runtime/vm/profiler.cc b/runtime/vm/profiler.cc
index e869f04802082edd177639565790d87fada7301b..0c17451e492541aeb16b4d311e1d52c1cf033411 100644
--- a/runtime/vm/profiler.cc
+++ b/runtime/vm/profiler.cc
@@ -84,8 +84,8 @@ void Profiler::InitOnce() {
 }
 
 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();
   }
 }