The method introduced by mruby#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)
```

dearblue added a commit to dearblue/mruby that referenced this pull request

If `Class#allocate` is prohibited, subclasses should also be implicitly prohibited.
```ruby
p Class.new(Struct).allocate.class
# => #<Class:0x82362ac00>                                             by mruby#6122
# => allocator undefined for #<Class:0x000000083a983220> (TypeError)  by Ruby 3.2
```
Added `MRB_DEFINE_ALLOCATOR()` to allow subclasses to use `Class#allocate`.
Supplement to mruby#6122.

Merged