um: vector: Avoid NULL ptr deference if transport is unset

When the transport option of a vec isn't set strncmp ends up being
called on a NULL pointer. Better not do that.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>