ddl: fix a bug in ddl close
ddl: fix a bug in ddl close

Fix a bug in https://github.com/pingcap/tidb/blob/master/ddl/ddl.go#L132.

tiancaiamao/tidb,8cbx/tidb,wusphinx/tidb,astaxie/tidb,qhsong/tidb,qiuyesuifeng/tidb,yuanwhy/tidb,c4pt0r/tidb,ngaut/tidb,EvilMcJerkface/tidb,qiuyesuifeng/tidb,c4pt0r/tidb,kyle-wang/tidb,JinheLin/tidb,cosmtrek/tidb,8cbx/tidb,wusphinx/tidb,anywhy/tidb,astaxie/tidb,anywhy/tidb,mccxj/tidb,kyle-wang/tidb,pingcap/tidb,spongedu/tidb,yangwenmai/tidb,kyle-wang/tidb,c4pt0r/tidb,zs634134578/tidb,kyle-wang/tidb,Nify/tidb,yuanwhy/tidb,liyu4/tidb,ngaut/tidb,pingcap/tidb,ming-relax/tidb,silenceper/tidb,XuHuaiyu/tidb,coocood/tidb,Cofyc/tidb,ueizhou/tidb,zimulala/tidb,kyle-wang/tidb,ueizhou/tidb,tiancaiamao/tidb,ngaut/tidb,spongedu/tidb,simon-xia/tidb,silentred/tidb,pingcap/tidb,Cofyc/tidb,ngaut/tidb,mccxj/tidb,coocood/tidb,myrice/tidb,cosmtrek/tidb,andelf/tidb,pingcap/tidb,liyu4/tidb,silenceper/tidb,yangwenmai/tidb,zimulala/tidb,c4pt0r/tidb,pingcap/tidb,ming-relax/tidb,JinheLin/tidb,louishust/tidb,qhsong/tidb,EvilMcJerkface/tidb,Nify/tidb,XuHuaiyu/tidb,pingcap/tidb,gaocegege/tidb,c4pt0r/tidb,kyle-wang/tidb,CodeRushing/tidb,andelf/tidb,louishust/tidb,c4pt0r/tidb,silentred/tidb,simon-xia/tidb,zs634134578/tidb,ngaut/tidb,CodeRushing/tidb,ngaut/tidb,gaocegege/tidb,myrice/tidb