Avoid corrupting its input in some cases.

BUG=none
TEST=test/mjsunit/lithium/StoreKeyed*.js
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/23600054

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16771 ce2b1a6d-e550-0410-aec6-3dcde31c8c00