InactiveReceiver panics on Drop

[Bruno Tavares]

When an InactiveReceiver is cloned, it does not increase the count of inactive receivers, and an error with the recent inactive receiver drop.

thread 'main' panicked at 'attempt to subtract with overflow', C:\Users\bltav\.cargo\registry\src\github.com-1ecc6299db9ec823\async-broadcast-0.3.1\src\lib.rs:1562:13

Reproduced on:

fn main() {
    let (_, active_receiver) = async_broadcast::broadcast::<()>(1);
    let inactive = active_receiver.deactivate();
    let inactive2 = inactive.clone();
    drop(inactive);
    drop(inactive2);
}