Amend check for unread client data in threadpool.

THD::NET will have unread data, in case client uses compression, and
wraps multiple commands into  a single compression packet

MariaDB C/C sends COM_STMT_RESET+COM_STMT_EXECUTE, and wraps it into
a single compressed packet, when compression is on, thus trying to use
compression and prepared statements against a threadpool-enabled server
will result into a hang, before this patch.