v8: fix --max_old_space_size=4096 integer overflow

See https://code.google.com/p/v8/issues/detail?id=3857 for the bug
report and https://codereview.chromium.org/897543002 for the CL.

PR-URL: https://github.com/iojs/io.js/pull/1166


Reviewed-By: Fedor Indutny <fedor@indutny.com>