Ctrl+D on REPL provokes exception

[gciatto]

Exception in thread "main" it.unibo.tuprolog.ui.repl.NullInputException: The standard input has been close unexpectedly
	at it.unibo.tuprolog.ui.repl.TuPrologUtils.readQuery(TuPrologUtils.kt:55)
	at it.unibo.tuprolog.ui.repl.TuPrologCmd.readEvalPrintLoop(TuPrologCmd.kt:101)
	at it.unibo.tuprolog.ui.repl.TuPrologCmd.run(TuPrologCmd.kt:59)
	at com.github.ajalt.clikt.parsers.Parser.parse(Parser.kt:170)
	at com.github.ajalt.clikt.parsers.Parser.parse(Parser.kt:16)
	at com.github.ajalt.clikt.core.CliktCommand.parse(CliktCommand.kt:258)
	at com.github.ajalt.clikt.core.CliktCommand.parse$default(CliktCommand.kt:255)
	at com.github.ajalt.clikt.core.CliktCommand.main(CliktCommand.kt:273)
	at com.github.ajalt.clikt.core.CliktCommand.main(CliktCommand.kt:298)
	at it.unibo.tuprolog.ui.repl.Main.main(Main.kt:8)