Fix panic on reconnect

[johanstokking]

We're getting a panic with ca94c53 (current master):

2020-09-17T13:10:34.159+02:00	panic: runtime error: invalid memory address or nil pointer dereference
2020-09-17T13:10:34.159+02:00	[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x1affabf]
2020-09-17T13:10:34.159+02:00	goroutine 1673161 [running]:
2020-09-17T13:10:34.159+02:00	github.com/eclipse/paho%2emqtt%2egolang.(*client).reconnect(0xc001db5680)
2020-09-17T13:10:34.159+02:00	/home/runner/go/pkg/mod/github.com/eclipse/paho.mqtt.golang@v1.2.1-0.20200609161119-ca94c5368c77/client.go:321 +0x31f
2020-09-17T13:10:34.159+02:00	created by github.com/eclipse/paho%2emqtt%2egolang.(*client).internalConnLost
2020-09-17T13:10:34.159+02:00	/home/runner/go/pkg/mod/github.com/eclipse/paho.mqtt.golang@v1.2.1-0.20200609161119-ca94c5368c77/client.go:459 +0x1f1

This is a regression introduced in #416 where the loop breaks while conn is unset. The log message can still be relevant for debugging.

@MattBrittan

[MattBrittan]

Sorry about introducing that one. I will reach out to @alsm again and see if we can get this committed (unfortunately there are a few outstanding issues; I'm currently running a forked version myself).