MDEV-23466 SIGABRT on SELECT WSREP_LAST_SEEN_GTID

SELECT WSREP_LAST_SEEN_GTID aborts the server if no provider is
loaded.

mysql-test/suite/wsrep/r/MDEV-23466.result

+SELECT WSREP_LAST_SEEN_GTID();
+WSREP_LAST_SEEN_GTID()
+00000000-0000-0000-0000-000000000000:-1

mysql-test/suite/wsrep/t/MDEV-23466.cnf

+!include ../my.cnf
+
+[mysqld.1]
+wsrep-on=OFF
+binlog-format=ROW
+wsrep-provider=none
+wsrep-cluster-address='gcomm://'
+innodb_autoinc_lock_mode=2

mysql-test/suite/wsrep/t/MDEV-23466.test

+#
+# MDEV-23466: SIGABRT in wsrep::server_state::provider on
+# SELECT WSREP_LAST_SEEN_GTID() on optimized builds
+#
+
+--source include/have_innodb.inc
+--source include/have_wsrep.inc
+--source include/have_binlog_format_row.inc
+
+SELECT WSREP_LAST_SEEN_GTID();

sql/item_strfunc.cc

@@ -5379,9 +5379,13 @@ String *Item_func_wsrep_last_written_gtid::val_str_ascii(String *str)
 
 String *Item_func_wsrep_last_seen_gtid::val_str_ascii(String *str)
 {
-  /* TODO: Should call Wsrep_server_state.instance().last_committed_gtid()
-     instead. */
-  wsrep::gtid gtid= Wsrep_server_state::instance().provider().last_committed_gtid();
+  wsrep::gtid gtid= wsrep::gtid::undefined();
+  if (Wsrep_server_state::instance().is_provider_loaded())
+  {
+    /* TODO: Should call Wsrep_server_state.instance().last_committed_gtid()
+       instead. */
+    gtid= Wsrep_server_state::instance().provider().last_committed_gtid();
+  }
   if (gtid_str.alloc(wsrep::gtid_c_str_len()))
   {
     my_error(ER_OUTOFMEMORY, wsrep::gtid_c_str_len());