Multithreaded TCPServer crashes immediately

[Gundolf68]

Was: mruby/mruby#4949

This:

server = TCPServer.open(8000)    
loop do                          
	Thread.start(server.accept) do |client|
		client.write "HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\nConnection: close\r\nContent-Length: 11\r\n\r\nHello world"
		client.close                  
	end
end

on Ubuntu 18.04.
At the first request, the program exits with a Segmentation fault.