setting max_allowed_packet size in ma_net_init() might end up in race condition

Thanks to Alexander Sapin for reporting this issue here:

Race condition

It might be a relict from old libmysql 3.23 code, but it is not necessary to adjust max_allowed_packet in every call of ma_net_init().
If necessary max_allowed_packet should be set once via mysql_optionsv (either globally or per connection).