client: avoid segfault in iwctl quit

iwctl quit (running quit non-interactively) isn't a useful command,
but it shouldn't segfault. Let's avoid calling readline functions if
we haven't initialized readline in this run.

client/display.c

@@ -890,7 +890,8 @@ void display_agent_prompt_release(const char *label)
 
 void display_quit(void)
 {
-	rl_crlf();
+	if (command_is_interactive_mode())
+		rl_crlf();
 }
 
 static void window_change_signal_handler(void *user_data)