fix: avoid pointer-arith error

[jeromelebleu]

This tries to fix the following errors:

ffi.c: In function ‘cdata_from_lua_table’:
ffi.c:859:50: error: pointer of type ‘void *’ used in arithmetic [-Werror=pointer-arith]
  859 |             cdata_from_lua(L, ct->array->ct, ptr + ctype_sizeof(ct->array->ct) * i++, lua_absindex(L, -1), cast);
      |                                                  ^
ffi.c:874:50: error: pointer of type ‘void *’ used in arithmetic [-Werror=pointer-arith]
  874 |                 cdata_from_lua(L, field->ct, ptr + field->offset, lua_absindex(L, -1), cast);
      |                                                  ^
ffi.c: In function ‘cdata_index_ptr’:
ffi.c:995:33: error: pointer of type ‘void *’ used in arithmetic [-Werror=pointer-arith]
  995 |         cdata_to_lua(L, ct, ptr + ctype_sizeof(ct) * idx);
      |                                 ^
ffi.c:1005:42: error: pointer of type ‘void *’ used in arithmetic [-Werror=pointer-arith]
 1005 |         return cdata_from_lua(L, ct, ptr + ctype_sizeof(ct) * idx, 3, false);
      |                                          ^
ffi.c: In function ‘cdata_index_crecord’:
ffi.c:1080:40: error: pointer of type ‘void *’ used in arithmetic [-Werror=pointer-arith]
 1080 |         cdata_to_lua(L, field->ct, ptr + offset);
      |                                        ^
ffi.c:1089:49: error: pointer of type ‘void *’ used in arithmetic [-Werror=pointer-arith]
 1089 |         return cdata_from_lua(L, field->ct, ptr + offset, 3, false);
      |