patch 9.1.1323: b:undo_ftplugin not executed when re-using buffer

Problem:  b:undo_ftplugin not executed when re-using buffer
          (archy3)
Solution: explicitly execute b:undo_ftplugin in buflist_new() when
          re-using the current buffer

fixes: #17113
closes: #17133

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1323,
 /**/
     1322,
 /**/