[avalanche] Fix a potential use after free issue

Summary:
There is a typo introduced in D10711. We need to access the proof associated with a peer after it gets deleted, so we copy the shared pointer. Unfortunately there is a place where the original pointer was used instead, causing a potential use after free issue.

Ref T1854.

Test Plan:

./contrib/teamcity/build-configurations.py build-asan

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D11026