This avoids bogus calls to Fixed*Array::cast() when FastElements-backed objects are empty (and thus backed by empty_fixed_array).

Review URL: https://chromiumcodereview.appspot.com/11299190
Patch from Adam Klein <adamk@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13071 ce2b1a6d-e550-0410-aec6-3dcde31c8c00