Remove `mrb_assert()` in `Module.nesting`
The following code was causing SIGSEGV:
```ruby
Module.method(:nesting).call
```
@@ -670,8 +670,7 @@ mrb_mod_s_nesting(mrb_state *mrb, mrb_value mod)
ary = mrb_ary_new(mrb);
proc = mrb->c->ci[-1].proc; /* callee proc */
- mrb_assert(!MRB_PROC_CFUNC_P(proc));
- while (proc) {
+ while (proc && !MRB_PROC_CFUNC_P(proc)) {
if (MRB_PROC_SCOPE_P(proc)) {
struct RClass *c2 = MRB_PROC_TARGET_CLASS(proc);
