Assert that frame() derefs a non-null pointer.

Source/core/editing/Editor.h

Index: Source/core/editing/Editor.h
diff --git a/Source/core/editing/Editor.h b/Source/core/editing/Editor.h
index a29a973e692fb99391d318093487bb7e2d2bbc1e..cb42d35c2a2b6d6d1b42ecdf10937de81dd147bc 100644
--- a/Source/core/editing/Editor.h
+++ b/Source/core/editing/Editor.h
@@ -141,7 +141,11 @@ public:
         // Returns 0 if this Command is not supported.
         int idForHistogram() const;
     private:
-        LocalFrame& frame() const { return *m_frame; }
+        LocalFrame& frame() const
+        {
+            ASSERT(m_frame);
+            return *m_frame;
+        }
 
         const EditorInternalCommand* m_command;
         EditorCommandSource m_source;
@@ -246,7 +250,11 @@ private:
 
     explicit Editor(LocalFrame&);
 
-    LocalFrame& frame() const { return *m_frame; }
+    LocalFrame& frame() const
+    {
+        ASSERT(m_frame);
+        return *m_frame;
+    }
 
     bool canDeleteRange(Range*) const;