Fix dangling pointer in custom Reference _new

[sheepandshepherd]

GODOT_CLASS's _new() frees the pointer it's supposed to return:

Name *instance = godot::as<Name>(script->new_());
//                │               │
//                │               └─> returns Variant: ref-count is 1
//                └─> returns Name*: ref-count drops to 0 when the Variant is destroyed

This PR inserts a reference() after the NativeScript::new_() call.

It does the same thing as #346, only at the location actually causing the dangling pointer, rather than in NativeScript::new_(). That function already works correctly, returning with a ref-count of 1.

Fixes #343 and #397

As discussed in #215, #346, and this comment, there's now a memory leak of the returned Reference because Ref(T *) increases its ref-count to 2. Another fix such as #350 will be needed in addition to this.

[BastiaanOlij]

@Zylann Is your PR an alternative to this or will this remain valid even after we apply your fix?

[Zylann]

@BastiaanOlij my fix solves the same problem without the side-effect, so I think it won't be needed anymore.