unix/modffi: Fix signed integer cast in return_ffi_value.

Casting an ffi_arg to a signed int may truncate the value. E.g., when the
ffi_arg is 64-bit and the signed int is 32-bit. Also, casting an ffi_arg
to a larger signed type will not sign extend the value. E.g., when the
ffi_arg is 32-bit and the larger signed type is int64_t. If the value is
signed, it should be cast to ffi_sarg, which is the same size as ffi_arg.

Signed-off-by: Michael Sawyer <mjfsawyer@gmail.com>

ports/unix/modffi.c

@@ -190,7 +190,7 @@ static mp_obj_t return_ffi_value(ffi_union_t *val, char type) {
         case 'h':
         case 'i':
         case 'l':
-            return mp_obj_new_int((signed)val->ffi);
+            return mp_obj_new_int((ffi_sarg)val->ffi);
         case 'B':
         case 'H':
         case 'I':