Fix re-initialization of existing double field.

R=verwaest@chromium.org
BUG=v8:2717
TEST=mjsunit/regress/regress-2717

Review URL: https://codereview.chromium.org/16735003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15033 ce2b1a6d-e550-0410-aec6-3dcde31c8c00