[ncp] initialize OT instance to nullptr in the constructor (#667)
[ncp] initialize OT instance to nullptr in the constructor (#667)


openthread/ot-br-posix,bukepo/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,bukepo/borderrouter,openthread/borderrouter,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,bukepo/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,bukepo/borderrouter,bukepo/borderrouter,openthread/borderrouter