Do not iterate past the end on processing leaf class.

tools/clang/blink_gc_plugin/tests/left_most_gc_base.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef LEFT_MOST_GC_BASE_H_
 #define LEFT_MOST_GC_BASE_H_
 
 #include "heap/stubs.h"
 
 namespace blink {
 
 class A { };
 class B { };
 
 class Right : public A, public B, public GarbageCollected<Right> { };  // Error
 class Left : public GarbageCollected<Left>, public B, public A { };
 
 class DerivedRight : public Right, public Left { };  // Error
 class DerivedLeft : public Left, public Right { };
 
+class C : public GarbageCollected<C> {
+public:
+    virtual void trace(Visitor*);
+};
+
+class IllFormed : public A, public C { }; // Error
+
 }
 
 #endif