resolve issue with arm code failing unit test

The optimizer did something funny with the code around
line 1412.  Before the call to encode_sb split_dist was
set properly but after it was adjusted and converted to
a negative.

https://code.google.com/p/webm/issues/detail?id=714

Change-Id: I9a7631d5325ade2dc28c1030653a23eecec8721b