Fix a potential overflow in SortedListBSearch

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/344513004

Patch from Jianghua Yang <jianghua.jhy@alibaba-inc.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21960 ce2b1a6d-e550-0410-aec6-3dcde31c8c00