net: Fix a potential segfault

net: Fix a potential segfault

If user forgets to provide any backend types for '-netdev' in qemu CLI,
It triggers seg fault.

e.g.

Expected:
$ qemu -netdev id=net0
qemu-system-x86_64: Parameter 'type' is missing

Actual:
$ qemu -netdev id=net0
Segmentation fault (core dumped)

Fixes: 547203ead4327 ("net: List available netdevs with "-netdev help")
Reviewed-by: Thomas Huth <thuth@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Lin Ma <lma@suse.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>