[avalanche] Don't consider our local proof as dangling

Summary: Our local proof might get evicted if no other node is using it because, as there it is treated as if there is no node attached. This diff fixes that by letting the cleanup function check for our local proof is there is one.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11745