usbip: vudc_tx: fix v_send_ret_submit() vulnerability to null xfer buffer

v_send_ret_submit() handles urb with a 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.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>