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/version.c b/src/version.c
index b25ed44..afb61e0 100644
--- a/src/version.c
+++ b/src/version.c
@@ -708,6 +708,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    294,
+/**/
     293,
 /**/
     292,