Possible fix for a crash

[or-else]

I2021/11/19 13:10:13 in: '{"sub":{"id":"95264","topic":"fnd","get":{"what":"sub"}}}' sid='RxIw50I5tpQ' uid='PFAQXh91150'
W2021/11/19 13:10:13 cluster: call failed tinode-2 unexpected EOF
W2021/11/19 13:10:13 proxy topic: route sess update request from proxy to master failed: unexpected EOF
I2021/11/19 13:10:14 cluster: connected to tinode-2
panic: sync: negative WaitGroup counter

goroutine 36840 [running]:
sync.(*WaitGroup).Add(0xc000480300, 0xc0004ce1a0)
	/usr/local/go/src/sync/waitgroup.go:74 +0x105
sync.(*WaitGroup).Done(...)
	/usr/local/go/src/sync/waitgroup.go:99
main.(*Topic).runProxy(0xc000480300, 0xc000cb9950)
	/Users/gene/go/src/github.com/tinode/chat/server/topic_proxy.go:41 +0x636
main.(*Topic).run(0xc0003fdad0, 0xc000cb9950)
	/Users/gene/go/src/github.com/tinode/chat/server/topic.go:208 +0x25
created by main.topicInit
	/Users/gene/go/src/github.com/tinode/chat/server/init_topic.go:128 +0x82f

[aforge]

Assuming all tests pass.

[or-else]

Yes, tests pass but only with MariaDB. MySQL has no arm64/v8 docker image.