filters_sse2.c: quiet integer sanitizer warnings
filters_sse2.c: quiet integer sanitizer warnings

missed in a788b49

with clang7+ quiets conversion warnings like:
implicit conversion from type 'int' of value -114 (32-bit, signed) to
type 'uint8_t' (aka 'unsigned char') changed the value to 142 (8-bit,
unsigned)

Change-Id: I52dcd9cd613107f5424177c277785b92430bffb7

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