AP_Scripting: MAVLink: fix segfault from trying to send on invalid channel

[IamPete1]

This fixes a segfault that can happen if you try and send a MAVLink message from scripting on a channel that does not exist.

In this case I was trying to broadcast a message on all channels from my script. Something like:

    for i = 0, 5 do
        mavlink:send_chan(i, id, encoded)
    end

Each commit fixes the issue on its own, but I think its a good idea to have both.

[magicrub]

The gcs file changes LGTM. I'll yield to @IamPete1 on the lua change

[rmackay9]

We agreed that @IamPete1 is free to merge after making the change that @timtuxworth has suggested regarding returning nil

[tpwrules]

LGTM