bit_writer_utils,Flush: quiet implicit conversion warnings
bit_writer_utils,Flush: quiet implicit conversion warnings

no change in object code

from clang-7 -fsanitize=implicit-integer-truncation
implicit conversion from type 'int32_t' (aka 'int') of value 287
(32-bit, signed) to type 'uint8_t' (aka 'unsigned char') changed the
value to 31 (8-bit, unsigned)

Change-Id: I692368bcc2f41412697b8ae51e53078831072891

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