Fix null pointer crash with adjustAndMark

R=ager@chromium.org, sigbjornf@opera.com
BUG=384246

Review URL: https://codereview.chromium.org/338533002

git-svn-id: svn://svn.chromium.org/blink/trunk@176166 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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