MDEV-23065 : Crash after setting wsrep_on to ON dynamically and reconnect

At end_connection make sure we have wsrep before trying to free
connection assigned to it.

mysql-test/suite/galera/r/galera#500.result

+connection node_1;
+connection node_2;
 connection node_2;
 SET SESSION wsrep_sync_wait = 0;
 SET GLOBAL wsrep_provider_options="gmcast.isolate=2";

mysql-test/suite/galera/t/galera#500.test

@@ -8,6 +8,11 @@
 --source include/galera_cluster.inc
 --source include/galera_have_debug_sync.inc
 
+# Save original auto_increment_offset values.
+--let $node_1=node_1
+--let $node_2=node_2
+--source include/auto_increment_offset_save.inc
+
 # Force node_2 gcomm background thread to terminate via exception.
 --connection node_2
 --let $wsrep_cluster_address = `SELECT @@wsrep_cluster_address`
@@ -36,3 +41,5 @@ SET SESSION wsrep_on=0;
 
 --connection node_2
 CALL mtr.add_suppression("WSREP: exception from gcomm, backend must be restarted: Gcomm backend termination was requested by setting gmcast.isolate=2.");
+
+--source include/auto_increment_offset_restore.inc

sql/sql_connect.cc

@@ -1112,7 +1112,7 @@ void end_connection(THD *thd)
 {
   NET *net= &thd->net;
 #ifdef WITH_WSREP
-  if (WSREP(thd))
+  if (WSREP(thd) && wsrep)
   {
     wsrep_status_t rcode= wsrep->free_connection(wsrep, thd->thread_id);
     if (rcode) {