AP_Scripting: prevent use of closed network socket

[tridge]

this gives a lua exception if you try to do this:

 sock:recv(1024)
 sock:close()
 sock:recv(1024)

without this we dereference null, causing a crash
initially noticed by @magicrub

[magicrub]

With this change, do we still need to set it to nil?

[tridge]

With this change, do we still need to set it to nil?

yes. This PR means you will get a scripting exception that will stop the script if you use a socket after closing it. Previously it could cause the board to reboot

[IamPete1]

I had a quick look if we could nul the object, but it seems not. https://stackoverflow.com/questions/49582545/set-lua-obj-to-nil-from-c