NetworkClient: initialize client_active and profilemanager_thread

StopClient guards profilemanager_thread and nulls it after the delete,
but nothing ever set it, so destroying a client that never started
read an uninitialized pointer. client_active was unset until
StartClient and is read on the same path.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f