usb: wusbcore: allow wa_xfer_destroy to clean up partially constructed xfers

If __wa_xfer_setup fails, it can leave a partially constructed wa_xfer
object.  The error handling code eventually calls wa_xfer_destroy which
does not check for NULL before dereferencing xfer->seg which could cause
a kernel panic.  This change also makes sure to free xfer->seg which was
being leaked for all transfers before this change.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>