Fix bogus assertion in SetFastDoubleElements.

R=danno@chromium.org
BUG=347530
LOG=N

Review URL: https://codereview.chromium.org/181433016

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19579 ce2b1a6d-e550-0410-aec6-3dcde31c8c00