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/message.c b/src/message.c
index e401882..b68c089 100644
--- a/src/message.c
+++ b/src/message.c
@@ -1157,6 +1157,10 @@
     // need_wait_return to do it later.
     if (msg_silent != 0)
 	return;
+#ifdef HAS_MESSAGE_WINDOW
+    if (in_echowindow)
+	return;
+#endif
 
     /*
      * When inside vgetc(), we can't wait for a typed character at all.