Fix LStoreKeyed error when tracing instruction.

LStoreKeyed can have a NULL value(), when we recognize we are storing a constant
hole value.

BUG=
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/15793006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14951 ce2b1a6d-e550-0410-aec6-3dcde31c8c00