browser_file: fix buffer issues with ReadAt
browser_file: fix buffer issues with ReadAt

Suggested by songgao.

Issue: #1817

keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client