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>