MDEV-7067: Server outputs Galera (WSREP) information, even if Galera is disabled

Additional changes :
* On startup, do not initialize wsrep if wsrep_on=0.
* On shutdown, stop wsrep replication only if > 0 wsrep
threads are running.

sql/mysqld.cc

@@ -1902,7 +1902,10 @@ static void __cdecl kill_server(int sig_ptr)
 #endif
 
   /* Stop wsrep threads in case they are running. */
+  if (wsrep_running_threads > 0)
+  {
     wsrep_stop_replication(NULL);
+  }
 
   close_connections();
 
@@ -5806,10 +5809,6 @@ int mysqld_main(int argc, char **argv)
       wsrep_create_appliers(wsrep_slave_threads - 1);
     }
   }
-  else
-  {
-    wsrep_init_startup (false);
-  }
 
  if (opt_bootstrap)
   {

sql/wsrep_var.cc

@@ -552,8 +552,8 @@ int wsrep_show_status (THD *thd, SHOW_VAR *var, char *buff,
     *v++= wsrep_status_vars[i];
 
   DBUG_ASSERT(i < maxi);
-  DBUG_ASSERT(wsrep != NULL);
 
+  if (wsrep != NULL) {
     wsrep_stats_var* stats= wsrep->stats_get(wsrep);
     for (wsrep_stats_var *sv= stats; i < maxi && sv && sv->name; i++, sv++, v++)
     {
@@ -575,6 +575,7 @@ int wsrep_show_status (THD *thd, SHOW_VAR *var, char *buff,
       DBUG_ASSERT(i < maxi);
     }
     wsrep->stats_free(wsrep, stats);
+  }
 
   my_qsort(buff, i, sizeof(*v), show_var_cmp);