Fix linux-user crashes in ioctl(SIOCGIFCONF) when ifc_buf is NULL.

Fix linux-user crashes in ioctl(SIOCGIFCONF) when ifc_buf is NULL.

Summary:
This is to fix bug https://bugs.launchpad.net/qemu/+bug/1796754.
It is valid for ifc_buf to be NULL according to
http://man7.org/linux/man-pages/man7/netdevice.7.html.

Signed-off-by: Kan Li <likan_999.student@sina.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20181024201303.114-1-likan_999.student@sina.com>
[lv: fix errors reported by checkpatch.pl]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>