Ncp: Fix join existing network feature (#865)

This commit changes the NcpBase and join-existing-network feature
by ensuring that the flag `mRequireJoinExistingNetwork` is not
cleared when role changes to detached or disabled. This fixes the
issue where a "join" command could seemingly succeed with device
forming its own partition (e.g, if a wrong network key is used).

src/ncp/ncp_base.cpp

@@ -740,7 +740,16 @@ void NcpBase::UpdateChangedProps(void)
         {
             if (mRequireJoinExistingNetwork)
             {
-                mRequireJoinExistingNetwork = false;
+                switch (otGetDeviceRole(mInstance))
+                {
+                case kDeviceRoleDetached:
+                case kDeviceRoleDisabled:
+                    break;
+
+                default:
+                    mRequireJoinExistingNetwork = false;
+                    break;
+                }
 
                 if ( (otGetDeviceRole(mInstance) == kDeviceRoleLeader)
                   && otIsSingleton(mInstance)