AU: Fix bsdiff hang

BUG=chrome-os-partner:6056
TEST=Manually create diff of obfuscated lib, apply diff and verify same
as target file, run cros_generate_update_payload to make sure it still
works

Change-Id: Ib0a68789fdf725d126467cbd86ca7ae28817b06c
Reviewed-on: http://gerrit.chromium.org/gerrit/8179


Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
Commit-Ready: Thieu Le <thieule@chromium.org>
Tested-by: Thieu Le <thieule@chromium.org>