[commissioner] only allow attached device to become commissioner (#4120)

The commit addresses the issue that running "commissioner start"
command when device role is detached will cause commissioner not be
able to start or stop.

src/core/meshcop/commissioner.cpp

@@ -144,6 +144,7 @@ otError Commissioner::Start(otCommissionerStateCallback  aStateCallback,
 {
     otError error = OT_ERROR_NONE;
 
+    VerifyOrExit(Get<Mle::MleRouter>().IsAttached(), error = OT_ERROR_INVALID_STATE);
     VerifyOrExit(mState == OT_COMMISSIONER_STATE_DISABLED, error = OT_ERROR_INVALID_STATE);
 
     SuccessOrExit(error = Get<Coap::CoapSecure>().Start(SendRelayTransmit, this));