usb-redirect: Avoid double free of data

If dropping packets, data is freed, the caller's loop should not continue.

Reported by ccc-analyzer.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1456301288-1592-1-git-send-email-famz@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
