MDEV-5523 Server crashes on DROP USER <rolename>

use 'user_name' for the error message (fixed, after get_current_user()),
not the original tmp_user_name, as it comes from the parser (host == NULL).

mysql-test/suite/roles/create_and_drop_role.result

@@ -36,3 +36,7 @@ select user, host, is_role from user where user like 'test%';
 user	host	is_role
 create role '';
 ERROR OP000: Invalid role specification ``.
+create role r1;
+drop user r1;
+ERROR HY000: Operation DROP USER failed for 'r1'
+drop role r1;

mysql-test/suite/roles/create_and_drop_role.test

@@ -52,3 +52,11 @@ connection default;
 --error ER_INVALID_ROLE
 create role '';
 
+#
+# MDEV-5523 Server crashes on DROP USER <rolename>
+#
+create role r1;
+--error ER_CANNOT_USER
+drop user r1;
+drop role r1;
+

sql/sql_acl.cc

@@ -9271,7 +9271,7 @@ bool mysql_drop_user(THD *thd, List <LEX_USER> &list, bool handle_as_role)
 
     if (handle_as_role != user_name->is_role())
     {
-      append_user(&wrong_users, tmp_user_name);
+      append_user(&wrong_users, user_name);
       result= TRUE;
       continue;
     }