netfilter: ip6t_srh: fix NULL pointer dereferences

netfilter: ip6t_srh: fix NULL pointer dereferences

[ Upstream commit 6d65561f3d5ec933151939c543d006b79044e7a6 ]

skb_header_pointer may return NULL. The current code dereference
its return values without a NULL check.

The fix inserts the checks to avoid NULL pointer dereferences.

Fixes: 202a8ff545cc ("netfilter: add IPv6 segment routing header 'srh' match")
Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin (Microsoft) <sashal@kernel.org>