gossip: Fix nr_live_nodes calculation
gossip: Fix nr_live_nodes calculation

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 437899909ddb4fe2c5a0814e553f84ca9c2d661f
(gossip: Talk to more live nodes in each gossip round).

Fixes #2637

Message-Id: <91ed81f37a53224ba0d39e0f2f5ca17a2ccd737e@scylladb.com>

duarten/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla