MDEV-8220 Server crashes if started with --enforce-storage-engine option

mysql-test/r/enforce_storage_engine_opt.result

+create table t1 (c1 int primary key auto_increment, c2 varchar(10)) engine=memory;
+Warnings:
+Note	1266	Using storage engine MyISAM for table 't1'
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `c1` int(11) NOT NULL AUTO_INCREMENT,
+  `c2` varchar(10) DEFAULT NULL,
+  PRIMARY KEY (`c1`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+set session sql_mode='no_engine_substitution';
+create table t2 (c1 int primary key auto_increment, c2 varchar(10)) engine=memory;
+ERROR 42000: Unknown storage engine 'MEMORY'
+drop table t1;

mysql-test/t/enforce_storage_engine_opt.opt

+--enforce-storage-engine=myisam

mysql-test/t/enforce_storage_engine_opt.test

+create table t1 (c1 int primary key auto_increment, c2 varchar(10)) engine=memory;
+show create table t1;
+set session sql_mode='no_engine_substitution';
+--error 1286
+create table t2 (c1 int primary key auto_increment, c2 varchar(10)) engine=memory;
+drop table t1;

sql/sql_plugin.cc

@@ -3107,6 +3107,9 @@ void plugin_thdvar_init(THD *thd)
     if (global_system_variables.tmp_table_plugin)
       thd->variables.tmp_table_plugin=
             intern_plugin_lock(NULL, global_system_variables.tmp_table_plugin);
+    if (global_system_variables.enforced_table_plugin)
+      thd->variables.enforced_table_plugin=
+            intern_plugin_lock(NULL, global_system_variables.enforced_table_plugin);
     intern_plugin_unlock(NULL, old_table_plugin);
     intern_plugin_unlock(NULL, old_tmp_table_plugin);
     intern_plugin_unlock(NULL, old_enforced_table_plugin);