MDEV-10041: Server crashes sporadically during bootstrap while running wsrep tests

The crash is caused due to a race condition where wsrep
startup threads invoke ha_maria::implicit_commit() method
while maria_hton is partially initialized. The fix is to
skip this method if plugins are uninitialized.

storage/maria/ha_maria.cc

@@ -2839,9 +2839,10 @@ int ha_maria::implicit_commit(THD *thd, bool new_trn)
   int error;
   uint locked_tables;
   DYNAMIC_ARRAY used_tables;
+  extern my_bool plugins_are_initialized;
   
   DBUG_ENTER("ha_maria::implicit_commit");
-  if (!maria_hton || !(trn= THD_TRN))
+  if (!maria_hton || !plugins_are_initialized || !(trn= THD_TRN))
     DBUG_RETURN(0);
   if (!new_trn && (thd->locked_tables_mode == LTM_LOCK_TABLES ||
                    thd->locked_tables_mode == LTM_PRELOCKED_UNDER_LOCK_TABLES))