fix fallback charset handling
fix fallback charset handling

test if the fallback is set, do not rely on error code
bookerzzz/mysql,VividCortex/mysql,siriele/mysql,muthusubbu459/mysql,rlowe/mysql,aperera/mysql,jockchou/mysql,segmentio/mysql,beego/mysql,helight/helight_code,hebihui/mysql,eXcomm/gomysql,helight/helight_code,hyperworks/mysql,eklitzke/mysql,helight/helight_code,bookerzzz/mysql,roth1002/mysql,go-sql-driver/mysql,netroby/mysql,zhgo/mysql,springning/mysql,Go-SQL-Driver/MySQL,helight/helight_code,helight/helight_code,cw2018/mysql,yyljlyy/mysql,helight/helight_code,methane/mysql,LightKool/mysql-go,tmulchay/mysql,sunjian1/mysql,joshuaprunier/mysql,helight/helight_code,Frca/mysql,cxflag203/mysql,henrylee2cn/mysql,nanata1115/mysql,methane/mysql,Vorstack/mysql,rafaeljusto/mysql,helight/helight_code,yonglehou/mysql,chinanjjohn2012/mysql,dalinaum/go-mysql-driver,helight/helight_code,VividCortex/mysql,segmentio/mysql,Freeaqingme/golang-sql-driver-mysql,wfxiang08/mysql,LightKool/mysql-go