Fix null pointer crash with adjustAndMark

Source/platform/heap/Visitor.h

Index: Source/platform/heap/Visitor.h
diff --git a/Source/platform/heap/Visitor.h b/Source/platform/heap/Visitor.h
index 15ec4047efd05d7e9dfec7f2ff92904d096cca11..56cfbef708c3273e745fb264ef1d08e098b867f4 100644
--- a/Source/platform/heap/Visitor.h
+++ b/Source/platform/heap/Visitor.h
@@ -621,7 +621,8 @@ class DefaultTraceTrait<T, true> {
 public:
     static void mark(Visitor* visitor, const T* self)
     {
-        self->adjustAndMark(visitor);
+        if (self)

[sof, 2014/06/13 09:12:54]

Would it be possible move this "up" a bit? i.e., we're paying for a null check
on pointers that are otherwise guaranteed to be non-null now, right?

[Erik Corry, 2014/06/13 09:40:26]

This is just for things that need adjustAndMark, so I'm not too worried about
the performance issues.  Also I think once stuff has been inlined any extra
checks are pretty simple to optimize out.

[sof, 2014/06/13 09:49:35]

alright, perhaps so.

+            self->adjustAndMark(visitor);
     }
 
 #ifndef NDEBUG