ipv6: xfrm: Handle errors reported by xfrm6_find_1stfragopt()

xfrm6_find_1stfragopt() may now return an error code and we must
not treat it as a length.

Fixes: 2423496a ("ipv6: Prevent overrun when parsing v6 header options")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Craig Gallek <kraig@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>