bgpd: Drop aggregator_as attribute if malformed in case of BGP_AS_ZERO

bgpd: Drop aggregator_as attribute if malformed in case of BGP_AS_ZERO

An UPDATE message that contains the AS number of zero in the AS_PATH
   or AGGREGATOR attribute MUST be considered as malformed and be
   handled by the procedures specified in [RFC7606].

An UPDATE message with a malformed AGGREGATOR attribute SHALL be
   handled using the approach of "attribute discard".

Attribute discard: In this approach, the malformed attribute MUST
      be discarded and the UPDATE message continues to be processed.
      This approach MUST NOT be used except in the case of an attribute
      that has no effect on route selection or installation.

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>