Frr/8.2

Why I did it Commercial Tool Code Check: The prev may exist as null

How I did it Add the prev is not null judgment

How to verify it Commercial tool testing passed

@ton31337 @donaldsharp help check and merge this, thanks!

What is the target at first? :) I think you wanted to contribute to frrouting/frr, not to sonic-net/sonic-frr, right?

What is the target at first? :) I think you wanted to contribute to frrouting/frr, not to sonic-net/sonic-frr, right?

The 8.2 branch of sonic-net/sonic-frr

@ton31337 @donaldsharp help check again and merge this, thanks!