Prohibit Class#allocate in a different way

The method introduced by #5979 causes a fault by swapping classes.

```console
% bin/mruby -e 'Method = Proc; p Object.method(:inspect)'
zsh: segmentation fault (core dumped)  bin/mruby -e 'Method = Proc; p Object.method(:inspect)'
```

After applying this patch, a `TypeError` exception will be raised.

```console
% bin/mruby -e 'Method = Proc; p Object.method(:inspect)'
trace (most recent call last):
        [1] -e:1
-e:1:in method: allocation failure of Proc (TypeError)
```

However, if the `mrb_vtype` is the same object, the same care must still be taken as before.

```console
% bin/mruby -e 'Method = Binding; p method(:puts).eval("12345")'
trace (most recent call last):
        [1] -e:1
-e:1:in eval: wrong argument type nil (expected Proc) (TypeError)
```

src/class.c

@@ -1949,10 +1949,10 @@ mrb_instance_alloc(mrb_state *mrb, mrb_value cv)
   else if (ttype == 0) {
     ttype = MRB_TT_OBJECT;
   }
+  if (MRB_UNDEF_ALLOCATOR_P(c)) {
+    mrb_raisef(mrb, E_TYPE_ERROR, "allocator undefined for %v", cv);
+  }
   if (ttype <= MRB_TT_CPTR) {
-    if (ttype == MRB_TT_UNDEF) {
-      mrb_raisef(mrb, E_TYPE_ERROR, "allocator undefined for %v", cv);
-    }
     mrb_raisef(mrb, E_TYPE_ERROR, "can't create instance of %v", cv);
   }
   o = (struct RObject*)mrb_obj_alloc(mrb, ttype, c);