ipv6: Don't pass invalid dst_entry pointer to dst_release().

Make sure dst_release() is not called with error pointer. This is
similar to commit 4910ac6c ("ipv4:
Don't ip_rt_put() an error pointer in RAW sockets.").

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>