bgpd: Ignore handling NLRIs if we received MP_UNREACH_NLRI

bgpd: Ignore handling NLRIs if we received MP_UNREACH_NLRI

If we receive MP_UNREACH_NLRI, we should stop handling remaining NLRIs if
no mandatory path attributes received.

In other words, if MP_UNREACH_NLRI received, the remaining NLRIs should be handled
as a new data, but without mandatory attributes, it's a malformed packet.

In normal case, this MUST not happen at all, but to avoid crashing bgpd, we MUST
handle that.

Reported-by: Iggy Frankovic <iggyfran@amazon.com>
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>