Improve the handling for invalid ipv4 address in admin_subfield

[Shivani-gslab]

so I understand this is how it behaves today but this feels wrong to me that we would silently ignore a potential input typo and replace it with router_id. Feels like this would be quite hard to tshoot.

Originally posted by @gmuloc in #5480 (comment)

[ClausHolbechArista]

agreed. In 6.0.0 let's raise an error in this case.