Corner case causes NextPacketID to never exit

[thedevop]

Step to reproduce:

• Call NextPacketID, current cl.State.packetID = 65534, inflight contains 1 to 65534, it successfully obtain 65535
• Call NextPacketID again, current cl.State.packetID = 65535, inflight contains 1 to 65535, it will never exit the loop

https://github.com/mochi-co/mqtt/blob/bb54cc68e67d43e1397286594541d77330b2910b/clients.go#L255-L256
If current value of cl.State.packetID is 65535, then i = 65535, started = 65536. Once it enters the for loop, i is set to 1 and overflowed = true. If inflight contains all 65535 messages, then the only condition it can exit the loop is:

		if overflowed && i == started {
			return 0, packets.ErrQuotaExceeded
		}

overflowed is true, but i will never reach 65536, i == started condition can never be met.

Here is an example in Go Playground, with the following condition removed.

   	if _, ok := cl.State.Inflight.Get(uint16(i)); !ok {
   		break
   	}

[thedevop]

Resolved in #169. Thanks @mochi-co.