patch 9.0.0789: dummy buffer ends up in a window

Problem:    Dummy buffer ends up in a window.
Solution:   Disallow navigating to a dummy buffer.

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    789,
 /**/
     788,
 /**/