Range#initialize_copy null pointer dereference

Heya! It's possible to segfault mruby through mruby-engine with the following snippet of code: Range.remove_method(:initialize_copy) (1..2).dup.to_s This can be triggered through mruby-engine like this: MRubyEngine.new(512*1024, 1000, 1000).sandbox_eval("/tmp", %{ Range.remove_method(:initialize_copy) (1..2).dup.to_s }) The `dup` and `clone` methods allocate a new object and then call `initialize_copy` on the new object with the old object as an argument to copy over internal state. Removing `Range#initialize_copy` makes it possible to construct an uninitialized `Range` object. Calling (pretty much) any instance method on the uninitialized `Range` object afterwards causes mruby to dereference a null pointer, leading to a segfault. I've attached a patch that fixes the bug by copying internal range state before calling `initialize_copy`, similar to what mruby already does for classes and modules.