[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>