bgpd: delete NULL assignment in bgp_attr_hash_alloc

bgpd: delete NULL assignment in bgp_attr_hash_alloc

If soft-reconfiguration is enabled, bgp_adj_in_set will be called
from bgp_update and bgp_adj_in_set will call bgp_attr_intern to intern
attr pointer. If given attr isn't found in attrhash, hash_get will call
bgp_attr_hash_alloc to allocate new attr structure. In
bgp_attr_hash_alloc, NULL will be assigned to srv6_vpn field and
srv6_l3vpn field in origin attr pointer. attr->srv6_vpn and
attr->srv6_l3vpn are interned in bgp_attr_intern, so NULL assignment
isn't needed.

And, these fields are used later in bgp_update to set SRv6 information
to bgp_path_info. If bgp_attr_hash_alloc assign NULL to these fields,
SRv6 information will be lost and incorrect routes are inserted into
data-plane.

Signed-off-by: Ryoga Saito <contact@proelbtn.com>