[PATCH] replication: fix a failing assert in replica_on_applier_disconnect()

[Serge Petrenko <sergepetrenko@tarantool.org>]

One possible case when two applier errors happen one after another
wasn't handled in replica_on_applier_disconnect(), which lead to
occasional test failures and crashes. Handle this case.

Part of #3510
---
This patch fixes an assertion fail, submitted by @locker in issue comments.
I wasn't able to reproduce 2 failures reported in the issue itself, and asked
for comments, but got no answer. I also couldn't fix the latter 2
failures just by looking at code.

https://github.com/tarantool/tarantool/tree/sergepetrenko/gh-3510-replication-asserts-fail
https://github.com/tarantool/tarantool/issues/3510

 src/box/replication.cc | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/box/replication.cc b/src/box/replication.cc
index 26bbbe32a..0efbd7c0e 100644
--- a/src/box/replication.cc
+++ b/src/box/replication.cc
@@ -350,6 +350,10 @@ replica_on_applier_disconnect(struct replica *replica)
 		assert(replicaset.applier.connected > 0);
 		replicaset.applier.connected--;
 		break;
+	case APPLIER_LOADING:
+		assert(replicaset.applier.loading > 0);
+		replicaset.applier.loading--;
+		break;
 	case APPLIER_DISCONNECTED:
 		break;
 	default:
-- 
2.15.2 (Apple Git-101.1)