[BUG] redis-plus-plus core dump / crash at AsyncRedisCluster reset

[jzkiss]

Describe the bug
AsyncRedisCluster reset causes coredump if one of the redis master was killed before.

To Reproduce
[1.] asynch client is defined / used in the following way:

::std::shared_ptr<::sw::redis::AsyncRedisCluster> m_redis_cluster;
m_redis_cluster.reset(new ::sw::redis::AsyncRedisCluster(opts, pool_opts, ::sw::redis::Role::MASTER));

[2.] Continuous traffic is generated

[3.] One redis master exits (kill -9 redis-server-pid or execute kubernetes rolling upgrade for the redis pods)

[4.] User code of redis-plus-plus detects that for 4 seconds there is no response for those requests that are directed to the unreachable redis (based on hash slot)

[5.] User code of redis-plus-plus initiates AsyncRedisCluster reset with ip-address / port of a reachable redis master
m_redis_cluster.reset(new ::sw::redis::AsyncRedisCluster(opts, pool_opts, ::sw::redis::Role::MASTER));

[6.] after a ~0.6 sec (restart: 14:59:39.710346104Z core dump: 14:59:40.398587602Z) core dump is detected:

[New LWP 1407]
[New LWP 1486]
[New LWP 1484]
[New LWP 1483]
[New LWP 1485]
[New LWP 1405]
[New LWP 1404]
[New LWP 1400]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Core was generated by `'.
Program terminated with signal SIGABRT, Aborted.
#0 0x0000000009625acf in raise () from /lib64/libc.so.6
[Current thread is 1 (Thread 0x122c1700 (LWP 1407))]
...
(gdb) bt full
#0 0x0000000009625acf in raise () from /lib64/libc.so.6
No symbol table info available.
#1 0x00000000095f8ea5 in abort () from /lib64/libc.so.6
No symbol table info available.
#2 0x0000000007e7d96a in uv_async_send.cold () from /lib64/libuv.so.1
No symbol table info available.
#3 0x0000000007c31856 in sw::redis::AsyncConnection::send (this=0xe82ee20, event=std::unique_ptrsw::redis::AsyncEvent = {...})
at /usr/include/c++/8/bits/shared_ptr_base.h:251
No locals.
#4 0x0000000007c42d96 in sw::redis::AsyncShardsPool::_redeliver_events (this=0xfdbbf90,
events=std::queue wrapping: std::deque with 6 elements = {...}) at /usr/include/c++/8/bits/move.h:74
async_event =
pool = std::shared_ptrsw::redis::AsyncConnectionPool (use count 3, weak count 1) = {get() = 0xfe40990}
connection = {_pool = std::shared_ptrsw::redis::AsyncConnectionPool (use count 3, weak count 1) = {get() = 0xfe40990},
_connection = std::shared_ptrsw::redis::AsyncConnection (use count 3, weak count 1) = {get() = 0xe82ee20}}
event =
should_stop_worker = false
#5 0x0000000007c44530 in sw::redis::AsyncShardsPool::_run (this=0xfdbbf90)
at /.../redis++/rpm/BUILD/src/sw/redis++/async_shards_pool.cpp:191
events = std::queue wrapping: std::deque with 6 elements = {{key = "USER_KEY_297",
event = std::unique_ptrsw::redis::AsyncEvent = {get() = 0x0}}, {key = "USER_KEY_302",
event = std::unique_ptrsw::redis::AsyncEvent = {get() = 0xe8b2d30}}, {
key = "USER_KEY_303", event = std::unique_ptrsw::redis::AsyncEvent = {
get() = 0xe8a38a0}}, {key = "USER_KEY_522",
event = std::unique_ptrsw::redis::AsyncEvent = {get() = 0xe8d7a60}}, {
key = "USER_KEY_306", event = std::unique_ptrsw::redis::AsyncEvent = {
get() = 0xe6d7e20}}, {key = "", event = std::unique_ptrsw::redis::AsyncEvent = {get() = 0xe6c8dc0}}}
#6 0x0000000008d6ab23 in execute_native_thread_routine () from /lib64/libstdc++.so.6
No symbol table info available.
#7 0x00000000083591ca in start_thread () from /lib64/libpthread.so.0
No symbol table info available.
#8 0x0000000009610e73 in clone () from /lib64/libc.so.6
No symbol table info available.
(gdb) Quit

USER_KEY_297, ..., USER_KEY_306 are anonymized keys, but all of them belongs to the slot range of the killed redis master

Expected behavior
No crash, traffic should be stabilized.

Environment:
OS: Rocky Linux 8.2-20.el8.0.1
Compiler: gcc version 8.5.0
hiredis version: hiredis 1.2.0
redis-plus-plus version: 1.3.12

Additional context
Redis cluster is used with 3 masters and 3 slaves.