test_bpf: Fix NULL vs IS_ERR() check in test_skb_segment()

The skb_segment() function returns error pointers on error.  It never
returns NULL.

Fixes: 76db8087 ("net: bpf: add a test for skb_segment in test_bpf module")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Yonghong Song <yhs@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>