usbip: fix stub_send_ret_submit() vulnerability to null transfer_buffer

usbip: fix stub_send_ret_submit() vulnerability to null transfer_buffer

commit be6123df1ea8f01ee2f896a16c2b7be3e4557a5a upstream.

stub_send_ret_submit() handles urb with a potential null transfer_buffer,
when it replays a packet with potential malicious data that could contain
a null buffer. Add a check for the condition when actual_length > 0 and
transfer_buffer is null.

Reported-by: Secunia Research <vuln@secunia.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>