gi: fix the usage of closure in CallbackIn::release

We were getting closure pointer from in_arg. Unfortunately, in_arg contains the native closure pointer, which does not equals to of the pointer to ffi_closure if libffi is built without --disable-exec-static-tramp.

This causes 46 test failures on a x86_64-linux with libffi built without --disable-exec-static-tramp. I guess gjs has never been really tested with such a libffi build :).

Fixes #479 (closed).