[NET]: Fix OOPS due to missing check in dev_parse_header().

[ This is kernel bugzilla 9174 "linux-2.6.23-git11 kernel panic" ]

The device in question is an IPv6-over-IPv4 tunnel, which doesn't have
any header_ops, so the crash happens in dev_parse_header when
dereferencing them.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -827,7 +827,7 @@ static inline int dev_parse_header(const struct sk_buff *skb,
 {
 	const struct net_device *dev = skb->dev;
 
-	if (!dev->header_ops->parse)
+	if (!dev->header_ops || !dev->header_ops->parse)
 		return 0;
 	return dev->header_ops->parse(skb, haddr);
 }