net.http: fix panic on empty addr, cleanup listen_and_serve Server method

[ttytm]

Fixes a panic when the server gets an empty address. Does a cleanup to the related scope.

import net.http

mut server := &http.Server{
	addr: ''
	on_running: fn (mut server http.Server) {
		spawn fn [mut server] () {
			println('server started')
			server.stop()
			println('server stopped')
		}()
	}
}
server.listen_and_serve()

Instead it will now behave like before passing 0 to the deprecated port field, or :0 to addr - using a random port.

[spytheman]

Excellent work.