We need to consider the _live_endpoints size. The nr_live_nodes should
not be larger than _live_endpoints size, otherwise the loop to collect
the live node can run forever.
It is a regression introduced in commit 437899909d
(gossip: Talk to more live nodes in each gossip round).
Fixes#2637
Message-Id: <863ec3890647038ae1dfcffc73dde0163e29db20.1501026478.git.asias@scylladb.com>