Store calculated setup_len in a local variable, verify it, and only
write it to the struct (USBDevice->setup_len) in case it passed the
sanity checks.

This prevents other code (do_token_{in,out} functions specifically)
from working with invalid USBDevice->setup_len values and overrunning
the USBDevice->setup_buf[] buffer.

Fixes: CVE-2020-14364
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-id: 20200825053636.29648-1-kraxel@redhat.com