bgpd: Set attr to NULL when passing NLRI_UPDATE with treat-as-withdraw

bgpd: Set attr to NULL when passing NLRI_UPDATE with treat-as-withdraw

Before this patch, we always passed `struct attr` for NLRI_UPDATE, but if we
have a situation with treat-as-withdraw (for example: malformed attribute, or
using a command like `neighbor path-attribute treat-as-withdraw`) the route
MUST be withdrawn form the BGP table.

Hence, we MUST pass attr as NULL, in this case we already have this check
under NLRI_ATTR_ARG() macro, just reuse it properly.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>