um: Remove redundant NULL check

Fix below warnings reported by coccicheck:
./arch/um/drivers/vector_user.c:403:2-7: WARNING: NULL check before some freeing functions is not needed.

Fixes: bc8f8e4e ("um: Add a generic "fd" vector transport")
Signed-off-by: Li Heng <liheng40@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>