lossless.c: remove U32 -> S8 conversion warnings
lossless.c: remove U32 -> S8 conversion warnings

Change-Id: Ica2664ea087254959391275654412141ed9472df

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