enb_registry: fix duplicate registration check in enb_register/0
next_handle was bound in the function head and reused in the case pattern {ok, Handle}, turning what looks like a binding into an equality test against next_handle. The duplicate check therefore only fired if the previously assigned handle happened to equal the current next_handle counter, which is essentially never true.
Fix by removing next_handle from the function head pattern and reading it with S#state.next_handle inside the error branch, so that Handle in {ok, Handle} is always a fresh binding that matches any existing handle.
