fix: unset Zeroconf user on restore error to allow takeover

[devgianlu]

Fixes #240, supersedes #258

If we fail to restore the previous session when a user disconnects, unset the current Zeroconf user so that it can login again.

Without this, we would just return without actually attempting:

go-librespot/zeroconf/zeroconf.go

Lines 185 to 194 in 84f1037

	if z.currentUser == username || z.authenticatingUser == username {
	z.userLock.Unlock()
	writer.WriteHeader(http.StatusOK)
	return json.NewEncoder(writer).Encode(AddUserResponse{
	Status: 101,
	StatusString: "OK",
	SpotifyError: 0,
	})
	}