MDEV-30133 MariaDB startup does not validate plugin-wsrep-provider when...

MDEV-30133 MariaDB startup does not validate plugin-wsrep-provider when wsrep_mode=off or wsrep_provider is not set

Refuse to start if WSREP_ON=OFF or wsrep_provider is not set or
it is set to 'none' if plugin-wsrep-provider is used.

mysql-test/suite/wsrep/r/wsrep_provider_plugin.result

@@ -4,7 +4,7 @@ SHOW VARIABLES LIKE 'wsrep_provider_repl_max_ws_size';
 Variable_name	Value
 wsrep_provider_repl_max_ws_size	1
 INSERT INTO t1 VALUES (1);
-ERROR HY000: Got error 5 "Input/output error" during COMMIT
+ERROR HY000: Maximum writeset size exceeded
 SET GLOBAL wsrep_provider_repl_max_ws_size=DEFAULT;
 SHOW VARIABLES LIKE 'wsrep_provider_repl_max_ws_size';
 Variable_name	Value
@@ -18,3 +18,6 @@ ERROR HY000: Variable 'wsrep_provider' is a read only variable
 DROP TABLE t1;
 CALL mtr.add_suppression("transaction size limit");
 CALL mtr.add_suppression("rbr write fail");
+SELECT VARIABLE_NAME,READ_ONLY FROM information_schema.system_variables where VARIABLE_NAME like '%wsrep_provider_options%';
+VARIABLE_NAME	READ_ONLY
+WSREP_PROVIDER_OPTIONS	YES

sql/wsrep_plugin.cc

@@ -276,6 +276,15 @@ void wsrep_destroy_sysvar(struct st_mysql_sys_var *var)
 static int wsrep_provider_plugin_init(void *p)
 {
   WSREP_DEBUG("wsrep_provider_plugin_init()");
+
+  if (!WSREP_ON_ || !wsrep_provider ||
+      (wsrep_provider && !strncasecmp(wsrep_provider, WSREP_NONE, FN_REFLEN)))
+  {
+	  WSREP_ERROR("plugin-wsrep-provider can't be enabled if wsrep_on==OFF or wsrep_provider is unset or set to 'none'");
+	  my_message(ER_WRONG_ARGUMENTS, "plugin-wsrep-provider can't be enabled if wsrep_on==OFF or wsrep_provider is unset or set to 'none'", MYF(0));
+	  return 1;
+  }
+
   provider_plugin_enabled= true;
 
   // When plugin-wsrep-provider is enabled we set