[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