Guard an unsafe cast of a catch context's extension object.

src/contexts.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 285 matching lines @@
   // because we frequently have to know if arbitrary objects are global
   // contexts.
   bool IsFunctionContext() {
     Map* map = this->map();
     return map == map->GetHeap()->function_context_map();
   }
   bool IsCatchContext() {
     Map* map = this->map();
     return map == map->GetHeap()->catch_context_map();
   }
+  bool IsWithContext() {
+    Map* map = this->map();
+    return map == map->GetHeap()->with_context_map();
+  }
 
   // Tells whether the global context is marked with out of memory.
   inline bool has_out_of_memory();
 
   // Mark the global context with out of memory.
   inline void mark_out_of_memory();
 
   // The exception holder is the object used as a with object in
   // the implementation of a catch block.
   bool is_exception_holder(Object* object) {
@@ skipping 79 matching lines @@
 #ifdef DEBUG
   // Bootstrapping-aware type checks.
   static bool IsBootstrappingOrContext(Object* object);
   static bool IsBootstrappingOrGlobalObject(Object* object);
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_CONTEXTS_H_