patch 9.0.0447: using :echowin while at the hit-enter prompt causes problems

Problem:    Using :echowin while at the hit-enter prompt causes problems.
Solution:   Do not prompt for :echowin.  Postpone showing the message window.
            Start the timer when the window is displayed.
diff --git a/src/ex_getln.c b/src/ex_getln.c
index d425b3e..f9226de 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -3890,7 +3890,8 @@
     void
 compute_cmdrow(void)
 {
-    if (exmode_active || msg_scrolled != 0)
+    // ignore "msg_scrolled" in update_screen(), it will be reset soon.
+    if (exmode_active || (msg_scrolled != 0 && !updating_screen))
 	cmdline_row = Rows - 1;
     else
 	cmdline_row = W_WINROW(lastwin) + lastwin->w_height