Fix compilation warnings

```
lua_cmsgpack.c: In function ‘mp_unpack_full’:
lua_cmsgpack.c:802:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
else if (offset > len)
^
lua_cmsgpack.c: In function ‘luaopen_create’:
lua_cmsgpack.c:899:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
for (i = 0; i < (sizeof(cmds)/sizeof(*cmds) - 1); i++) {
^
lua_cmsgpack.c: In function ‘luaopen_cmsgpack_safe’:
lua_cmsgpack.c:934:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
for (i = 0; i < (sizeof(cmds)/sizeof(*cmds) - 1); i++) {
^
```