MDEV-3834 Crossgrade from MySQL 5.6.7 to MariaDB 10.0 fails due to non-existing mysql.host table

Treat the host table as optional, don't abort when it's missing

sql/sql_acl.cc

@@ -750,11 +750,13 @@ static my_bool acl_load(THD *thd, TABLE_LIST *tables)
   acl_cache->clear(1);				// Clear locked hostname cache
 
   init_sql_alloc(&mem, ACL_ALLOC_BLOCK_SIZE, 0);
+  (void) my_init_dynamic_array(&acl_hosts,sizeof(ACL_HOST),20,50);
+  if (tables[0].table) // "host" table may not exist (e.g. in MySQL 5.6.7+)
+  {
     if (init_read_record(&read_record_info, thd, table= tables[0].table,
                          NULL, 1, 1, FALSE))
       goto end;
     table->use_all_columns();
-  (void) my_init_dynamic_array(&acl_hosts,sizeof(ACL_HOST),20,50);
     while (!(read_record_info.read_record(&read_record_info)))
     {
       ACL_HOST host;
@@ -799,6 +801,7 @@ static my_bool acl_load(THD *thd, TABLE_LIST *tables)
     my_qsort((uchar*) dynamic_element(&acl_hosts,0,ACL_HOST*),acl_hosts.elements,
              sizeof(ACL_HOST),(qsort_cmp) acl_compare);
     end_read_record(&read_record_info);
+  }
   freeze_size(&acl_hosts);
 
   if (init_read_record(&read_record_info, thd, table=tables[1].table,
@@ -1173,7 +1176,7 @@ my_bool acl_reload(THD *thd)
   tables[2].next_local= tables[2].next_global= tables + 3;
   tables[0].open_type= tables[1].open_type= tables[2].open_type= 
   tables[3].open_type= OT_BASE_ONLY;
-  tables[3].open_strategy= TABLE_LIST::OPEN_IF_EXISTS;
+  tables[0].open_strategy= tables[3].open_strategy= TABLE_LIST::OPEN_IF_EXISTS;
 
   if (open_and_lock_tables(thd, tables, FALSE, MYSQL_LOCK_IGNORE_TIMEOUT))
   {