Fix a bug where CDC ACM BufferedReceiver repeats data when its future is dropped

[chrisdoble]

This PR fixes a bug where the CDC ACM BufferedReceiver can repeat previously received data when its read future is dropped.

Under the current implementation of read, if there's no buffered data and the provided buffer isn't large enough to contain an entire packet, self.start is set to 0 before awaiting a new packet while self.end is only updated after a packet arrives. If the read future is dropped at the await point, start has been reset to 0 but end retains its old value, causing the next call to read to repeat the previously received packet.

A scenario where you might run into this bug (where I ran into it) is if you want to read with a timeout, e.g.

with_timeout(Duration::from_secs(1), buffered_receiver.read(&mut buf))

If the read times out, the read future is dropped mid-execution and the BufferedReceiver is left in an inconsistent state.

My fix is to reorder the assignments so they're only updated after receiving a new packet.