src: use signed char for negative values
src: use signed char for negative values

PR-URL: https://github.com/bnoordhuis/node-buffertools/pull/76
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>

bnoordhuis/node-buffertools,bnoordhuis/node-buffertools,bnoordhuis/node-buffertools,bnoordhuis/node-buffertools