Fix calling Send using UM instance from UM hook

[Yarukon]

This should fix the use case below

public HookResult Hook_WeaponFiring(UserMessage userMessage)
{
    var weaponid = userMessage.ReadUInt("weapon_id");
    var soundType = userMessage.ReadInt("sound_type");
    var itemdefindex = userMessage.ReadUInt("item_def_index");

    userMessage.SetUInt("weapon_id", 0);
    userMessage.SetInt("sound_type", 9);
    userMessage.SetUInt("item_def_index", 61);

    // send for people who use silencer.
    userMessage.Recipients = new(silencer);
    userMessage.Send();

    userMessage.SetUInt("weapon_id", weaponid);
    userMessage.SetInt("sound_type", soundType);
    userMessage.SetUInt("item_def_index", itemdefindex);

    userMessage.Recipients = new(normal);

    return HookResult.Continue;
}

Code is from oylsister/StopSound@95998e3