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

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index f1bebf44bd4532b4919190efbd9a14492b9b90c0..100e1a256e689167dc2176f90935c5e2eb90ca17 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -303,6 +303,10 @@ class Context: public FixedArray {
     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();