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>

arch/um/drivers/vector_kern.c

@@ -197,6 +197,9 @@ static int get_transport_options(struct arglist *def)
 	long parsed;
 	int result = 0;
 
+	if (transport == NULL)
+		return -EINVAL;
+
 	if (vector != NULL) {
 		if (kstrtoul(vector, 10, &parsed) == 0) {
 			if (parsed == 0) {