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/gc.c

@@ -476,7 +476,6 @@ mrb_obj_alloc(mrb_state *mrb, enum mrb_vtype ttype, struct RClass *cls)
     }
     tt = MRB_INSTANCE_TT(cls);
     if (tt != MRB_TT_FALSE &&
-        tt != MRB_TT_UNDEF &&
         ttype != MRB_TT_SCLASS &&
         ttype != MRB_TT_ICLASS &&
         ttype != MRB_TT_ENV &&