slirp: Keep next_m always valid

Make sure that next_m always points to a packet if batchq is non-empty.
This will simplify walking the queues in if_start.

CC: Fabien Chouteau <chouteau@adacore.com>
CC: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
CC: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
