[IPV4]: Remove dead code from ip_output.c

skb_prev is assigned from skb, which cannot be NULL. This patch removes the
unnecessary NULL check.

Signed-off-by: Jayachandran C. <c.jayachandran at gmail.com>
Acked-by: James Morris <jmorris@namei.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>