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.
diff --git a/src/version.c b/src/version.c
index 1c511f9..05e619f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    789,
+/**/
     788,
 /**/
     787,