optimize_b_greedy: Avoid left shift of negative value.

Before:
-------
accu_error was always <= 0:
- It was initialized to zero, and
- Became more and more negative through the iterations.

This meant that RDOPT() macro usage on accu_error was causing left shift
of negative value.

Now:
----
- accu_error is initialized to a large positive value.
- It becomes smaller on every iteration but remains >= always.

This is functionally equivalent: verified that there's no change in
output due to this patch.

BUG=aomedia:600

Change-Id: I654572a72234c588c8f332d5b66a5587fc6610b4