patch 9.0.0308: when cmdheight is zero the attention prompt doesn't show

Problem:    When cmdheight is zero the attention prompt doesn't show.
Solution:   Do not use the message window for a prompt.
diff --git a/src/memline.c b/src/memline.c
index a495636..3911213 100644
--- a/src/memline.c
+++ b/src/memline.c
@@ -4640,6 +4640,7 @@
     stat_T	st;
     time_t	swap_mtime;
 
+    dont_use_message_window();
     ++no_wait_return;
     (void)emsg(_(e_attention));
     msg_puts(_("\nFound a swap file by the name \""));