This avoids casting problems in case of empty_fixed_array as
FixedDoubleArray with size 0. This avoids prematurely shortcutting in
case of 0 all over the place, which in turn leads to errors.

Review URL: https://chromiumcodereview.appspot.com/11299226

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13098 ce2b1a6d-e550-0410-aec6-3dcde31c8c00