Fixed assert in WSREP if one started with --wsrep_provider=.. --wsrep_on=OFF

Assert was:
mariadbd: /my/maria-10.6/wsrep-lib/src/client_state.cpp:256:
int wsrep::client_state::after_statement(): Assertion `state() == s_exec'

The reason was because of two faults:
- A missing test for WSREP(thd) when checking wsrep_after_statement(()
- THD->wsrep_cs().state was set to s_idle instead of s_none

sql/sql_parse.cc

@@ -3781,7 +3781,8 @@ mysql_execute_command(THD *thd)
     thd->transaction.stmt.mark_trans_did_ddl();
 #ifdef WITH_WSREP
     /* Clean up the previous transaction on implicit commit */
-    if (wsrep_thd_is_local(thd) && wsrep_after_statement(thd))
+    if (WSREP_NNULL(thd) && wsrep_thd_is_local(thd) &&
+        wsrep_after_statement(thd))
     {
       goto error;
     }
@@ -3855,7 +3856,7 @@ mysql_execute_command(THD *thd)
     Do not start transaction for stored procedures, it will be handled
     internally in SP processing.
   */
-  if (WSREP(thd)                          &&
+  if (WSREP_NNULL(thd)                    &&
       wsrep_thd_is_local(thd)             &&
       lex->sql_command != SQLCOM_BEGIN    &&
       lex->sql_command != SQLCOM_CALL     &&

sql/wsrep_trans_observer.h

@@ -407,8 +407,10 @@ static inline void wsrep_after_apply(THD* thd)
 static inline void wsrep_open(THD* thd)
 {
   DBUG_ENTER("wsrep_open");
-  if (WSREP_PROVIDER_EXISTS)
+  if (WSREP_ON_)
   {
+    /* WSREP_PROVIDER_EXISTS_ cannot be set if WSREP_ON_ is not set */
+    DBUG_ASSERT(WSREP_PROVIDER_EXISTS_);
     thd->wsrep_cs().open(wsrep::client_id(thd->thread_id));
     thd->wsrep_cs().debug_log_level(wsrep_debug);
     if (!thd->wsrep_applier && thd->variables.wsrep_trx_fragment_size)