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.

src/version.c

@@ -707,6 +707,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    294,
 /**/
     293,
 /**/