MDEV-6290 Crash in KILL HARD QUERY USER x@y when slave threads are running

KILL USER should ignore system threads where sctx->user=sctx->host=NULL

mysql-test/suite/rpl/r/kill_hard-6290.result

+include/master-slave.inc
+[connection master]
+kill user test2@nohost;
+include/rpl_end.inc

mysql-test/suite/rpl/t/kill_hard-6290.test

+#
+# MDEV-6290 Crash in KILL HARD QUERY USER x@y when slave threads are running
+#
+
+# this test doesn't depend on the binlog format, no need to run it three times
+--source include/have_binlog_format_mixed.inc
+
+--source include/master-slave.inc
+--connection server_2
+kill user test2@nohost;
+--source include/rpl_end.inc

sql/sql_parse.cc

@@ -7365,7 +7365,7 @@ static uint kill_threads_for_user(THD *thd, LEX_USER *user,
   I_List_iterator<THD> it(threads);
   while ((tmp=it++))
   {
-    if (tmp->command == COM_DAEMON)
+    if (!tmp->security_ctx->user)
       continue;
     /*
       Check that hostname (if given) and user name matches.