Fix assert failure in array slice

R=verwaest@chromium.org
BUG=

Review URL: https://codereview.chromium.org/11412005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12976 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/elements.cc

@@ -696,7 +696,7 @@ class ElementsAccessorBase : public ElementsAccessor {
         }
       }
     }
-    if (from->length() == 0) {
+    if (from->length() == 0 || copy_size == 0) {
       return from;
     }
     return ElementsAccessorSubclass::CopyElementsImpl(