SpinelNCPTaskLeave: After `leave` clear `mNetworkKey` and `mNetworkKeyIndex`
SpinelNCPTaskLeave: After `leave` clear `mNetworkKey` and `mNetworkKeyIndex`

d-bahr/wpantund,d-bahr/wpantund,d-bahr/wpantund,openthread/wpantund,abtink/wpantund,abtink/wpantund,openthread/wpantund,d-bahr/wpantund,aeliot/wpantund,aeliot/wpantund,aeliot/wpantund,aeliot/wpantund,abtink/wpantund,openthread/wpantund,openthread/wpantund,abtink/wpantund