Fix for auto decon of miners

[vfinn]

This is simpler code that works with any size mining field. The key is that miner.mining_target goes to nil when the miner.mining_target.amount == 0.

function OarcAutoDeconOnTick()
    if (global.oarc_decon_miners and (#global.oarc_decon_miners > 0)) then
        for i,miner in pairs(global.oarc_decon_miners) do
            if (miner and miner.valid and (miner.mining_target == nil)) then
                miner.order_deconstruction(miner.force)
            end
            table.remove(global.oarc_decon_miners, i)
        end
    end
end

MINOR ISSUE ;)