[avalanche] Make sure the round variable is initialized properly
Summary: As per title. The value of round is undefined without it and we'd like to avoid undefined behavior.
Test Plan:
make check
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2147