tidb-server: Limit salt to ascii chars
tidb-server: Limit salt to ascii chars

Fix JDBC auth fail bug.

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