runtime: Set b:undo_ftplugin where missing (#12943)

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/ftplugin/asm.vim b/runtime/ftplugin/asm.vim
index 0914bf6..f6a92d5 100644
--- a/runtime/ftplugin/asm.vim
+++ b/runtime/ftplugin/asm.vim
@@ -1,11 +1,13 @@
 " Vim filetype plugin file
 " Language:	asm
 " Maintainer:	Colin Caine <cmcaine at the common googlemail domain>
-" Last Changed: 23 May 2020
+" Last Change:  23 May 2020
+" 		2023 Aug 28 by Vim Project (undo_ftplugin)
 
 if exists("b:did_ftplugin") | finish | endif
+let b:did_ftplugin = 1
 
 setl comments=:;,s1:/*,mb:*,ex:*/,://
 setl commentstring=;%s
 
-let b:did_ftplugin = 1
+let b:undo_ftplugin = "setl commentstring< comments<"