Fix possible crash in SafeStackFrameIterator

src/frames.cc

Index: src/frames.cc
diff --git a/src/frames.cc b/src/frames.cc
index dfaf3ca5e4d6e838caf31913e62d46ec88375f10..5297bbbbe5a08ce627e4d85a55808ac9d55b6d1d 100644
--- a/src/frames.cc
+++ b/src/frames.cc
@@ -121,15 +121,15 @@ StackFrame* StackFrameIteratorBase::SingletonFor(StackFrame::Type type,
 
 StackFrame* StackFrameIteratorBase::SingletonFor(StackFrame::Type type) {
 #define FRAME_TYPE_CASE(type, field) \
-  case StackFrame::type: result = &field##_; break;
+  case StackFrame::type:             \
+    return &field##_;
 
-  StackFrame* result = NULL;
   switch (type) {
     case StackFrame::NONE: return NULL;
     STACK_FRAME_TYPE_LIST(FRAME_TYPE_CASE)
     default: break;
   }
-  return result;
+  return NULL;
 
 #undef FRAME_TYPE_CASE
 }
@@ -234,7 +234,7 @@ SafeStackFrameIterator::SafeStackFrameIterator(
   }
   if (SingletonFor(type) == NULL) return;
   frame_ = SingletonFor(type, &state);
-  if (frame_ == NULL) return;
+  DCHECK(frame_);

[titzer, 2016/01/27 08:29:00]

Did you run this through the trybots yet? I might have hit this one last time
when adding support for WASM frames. Is it safe(r) to just return if the frame
type is not known?

[alph, 2016/01/27 17:37:14]

AFAICT it cannot return NULL if the previous call was not null.
Dry run passed. I'll try to land it and keep watching it.

 
   Advance();
 
@@ -272,8 +272,12 @@ void SafeStackFrameIterator::AdvanceOneFrame() {
   // Advance to the previous frame.
   StackFrame::State state;
   StackFrame::Type type = frame_->GetCallerState(&state);
+  if (SingletonFor(type) == NULL) {

[yurys, 2016/01/26 23:55:57]

Maybe if (type < 0 || type >= StackFrame::NUMBER_OF_TYPES) ?

[alph, 2016/01/27 00:19:36]

I like mine more. Moreover it is used elsewhere (line 235).

[yurys, 2016/01/27 00:22:14]

Acknowledged.

+    frame_ = NULL;
+    return;
+  }
   frame_ = SingletonFor(type, &state);
-  if (frame_ == NULL) return;
+  DCHECK(frame_);
 
   // Check that we have actually moved to the previous frame in the stack.
   if (frame_->sp() < last_sp || frame_->fp() < last_fp) {