[avalanche] Fix an always true assertion in abc_p2p_getavaaddr

Summary: There is a loop intended to check that we get one more message after each iteration, but it's always true after the first one due to poor condition check.

Test Plan:

./test/functional/test_runner.py abc_p2p_getavaaddr

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11292