panic: connection error

#### System configuration
**Pulsar version**: 2.7.2
**Pulsar client version**: 0.5.0

`{"consumerID":13,"error":"connection error","level":"error","msg":"[Failed to create consumer]","name":"jpucv","subscription":"0","time":"2021-06-22T02:43:40Z","topic":"non-persistent://public/default/mqtt/broadcast-partition-15"}

{"consumerID":13,"error":"connection error","level":"error","msg":"[Failed to create consumer]","name":"jpucv","subscription":"0","time":"2021-06-22T02:43:40Z","topic":"non-persistent://public/default/mqtt/broadcast-partition-15"}

{"consumerID":3,"error":"connection error","level":"error","msg":"[Failed to create consumer]","name":"jpucv","subscription":"0","time":"2021-06-22T02:43:40Z","topic":"non-persistent://public/default/mqtt/broadcast-partition-12"}

{"consumerID":3,"error":"connection error","level":"error","msg":"[Failed to create consumer]","name":"jpucv","subscription":"0","time":"2021-06-22T02:43:40Z","topic":"non-persistent://public/default/mqtt/broadcast-partition-12"}

panic: connection error`

Start with the Go consumer-creation path that emits "Failed to create consumer" and the connection error shown in the report. Reproduce the failure with Pulsar 2.7.2 and client 0.5.0 using the logged topics, then determine the conditions that lead to the panic; done means the failure is explained and covered by an appropriate regression test.

Written by the indexing model from the issue text.