Fix MSAN error on arm64 bot.

src/profiler/tick-sample.cc

 // Copyright 2013 the V8 project 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 "src/profiler/tick-sample.h"
 
 #include "src/frames-inl.h"
 #include "src/vm-state-inl.h"
 
 
@@ skipping 107 matching lines @@
 bool TickSample::GetStackSample(Isolate* isolate, const v8::RegisterState& regs,
                                 RecordCEntryFrame record_c_entry_frame,
                                 void** frames, size_t frames_limit,
                                 v8::SampleInfo* sample_info) {
   sample_info->frames_count = 0;
   sample_info->vm_state = isolate->current_vm_state();
   sample_info->external_callback_entry = nullptr;
   if (sample_info->vm_state == GC) return true;
 
   Address js_entry_sp = isolate->js_entry_sp();
-  if (js_entry_sp == 0) return true;  // Not executing JS now.
+  if (js_entry_sp == nullptr) return true;  // Not executing JS now.
+  DCHECK(regs.sp);
 
   if (regs.pc && IsNoFrameRegion(static_cast<Address>(regs.pc))) {
     // Can't collect stack.
     return false;
   }
 
   ExternalCallbackScope* scope = isolate->external_callback_scope();
   Address handler = Isolate::handler(isolate->thread_local_top());
   // If there is a handler on top of the external callback scope then
   // we have already entrered JavaScript again and the external callback
@@ skipping 78 matching lines @@
     // it is not guaranteed to be atomic even when both host and target
     // are of same bitness.
     return false;
   }
   return true;
 }
 #endif  // USE_SIMULATOR
 
 }  // namespace internal
 }  // namespace v8