um: vector: Fix a memory allocation check

Checking the result of the previous 'kstrdup()' call is expected here, so
we should test 'params' and not 'str' (which is known to be non-NULL at
this point)

Fixes: ad1f62ab2bd4 ("High Performance UML Vector Network Driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/vector_kern.c

@@ -723,7 +723,7 @@ static int vector_config(char *str, char **error_out)
 	 */
 
 	params = kstrdup(params, GFP_KERNEL);
-	if (str == NULL) {
+	if (params == NULL) {
 		*error_out = "vector_config failed to strdup string";
 		return -ENOMEM;
 	}