MDEV-5408: Crash in mariadb-wsrep during plugin load at startup

Analysis: If wsrep_provider is not specified ("none") the wsrep is not
initialized.

Fix: Add additional guard not to call wsrep->post_rollback if 
wsrep_provider is not specified.

sql/wsrep_hton.cc

@@ -248,11 +248,13 @@ int wsrep_commit(handlerton *hton, THD *thd, bool all)
         Transaction didn't go through wsrep->pre_commit() so just roll back
         possible changes to clean state.
       */
-      if (wsrep->post_rollback(wsrep, &thd->wsrep_trx_handle))
-      {
-        DBUG_PRINT("wsrep", ("setting rollback fail"));
-        WSREP_ERROR("settting rollback fail: thd: %llu SQL: %s", 
+      if (WSREP_PROVIDER_EXISTS) {
+          if (wsrep->post_rollback(wsrep, &thd->wsrep_trx_handle))
+	  {
+              DBUG_PRINT("wsrep", ("setting rollback fail"));
+              WSREP_ERROR("settting rollback fail: thd: %llu SQL: %s",
                     (long long)thd->real_id, thd->query());
+	  }
       }
       wsrep_cleanup_transaction(thd);
     }