patch 9.0.0550: crash when closing a tabpage and buffer is NULL

Problem:    Crash when closing a tabpage and buffer is NULL.
Solution:   Adjust how autocommands are triggered when closing a window.
            (closes #11198, closes #11197)
diff --git a/src/version.c b/src/version.c
index e3a3a0e..5d6951c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    550,
+/**/
     549,
 /**/
     548,