fixed unused variables/code warnings in scale box function BUG=libyuv:432 TESTED=local windows build with chromium_code =1 R=bcornell@google.com
fixed unused variables/code warnings in scale box function
BUG=libyuv:432
TESTED=local windows build with chromium_code =1
R=bcornell@google.com

Review URL: https://webrtc-codereview.appspot.com/49849004
tony2012/libyuv,tony2012/libyuv,tony2012/libyuv