net: Fix -net socket,listen (Jan Kiszka)

In case no symbolic name is provided when requesting VLAN connection via
listening TCP socket ('-net socket,listen=...'), qemu crashes. This
fixes the cause.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7196 c046a42c-6fe2-441c-8c8c-71466251a162
