Fix compilation warnings

[antirez]

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++) {
                   ^