vm.c: fix integer type error in `mrb_protect_error`.
@@ -312,7 +312,7 @@ mrb_protect_error(mrb_state *mrb, mrb_protect_error_func *body, void *userdata,
mrb_value result = mrb_nil_value();
int ai = mrb_gc_arena_save(mrb);
const struct mrb_context *c = mrb->c;
- int ci_index = c->ci - c->cibase;
+ ptrdiff_t ci_index = c->ci - c->cibase;
if (error) { *error = FALSE; }
