nouveau/u_memcpya: fix NULL vs error pointer bug

The u_memcpya() function is supposed to return error pointers on
error. Returning NULL will lead to an Oops.

Fixes: e3885f71

 ("nouveau/u_memcpya: use vmemdup_user")
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/10fd258b-466f-4c5b-9d48-fe61a3f21424@moroto.mountain