patch 9.0.0294: crash when 'cmdheight' is 0 and popup_clear() used

Problem:    Crash when 'cmdheight' is 0 and popup_clear() used.
Solution:   Reset "message_win" when the message popup is cleared.  Close the
            popup when 'cmdheight' is non-zero.  Add a screendump test.
diff --git a/src/proto/popupwin.pro b/src/proto/popupwin.pro
index ef76941..a12cb3e 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -66,6 +66,7 @@
 void popup_show_message_win(void);
 int popup_message_win_visible(void);
 void popup_hide_message_win(void);
+void popup_close_message_win(void);
 int popup_win_closed(win_T *win);
 void popup_set_title(win_T *wp);
 void popup_update_preview_title(void);