Fix an unsigned integer overflow error in enc/cost.h
Fix an unsigned integer overflow error in enc/cost.h

Change-Id: I9774b59c417c185f09a61a115364b9642976a100

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp