Fix topic copying for chunked messages

[mkfrey]

Only copy the topic on the first message chunk. Otherwise the old copy will be
destroyed on subsequent calls to _onMqttMessage() for the same message, while
the copy is still being referenced by _mqttTopicLevels. This leads to undefined
behaviour.

Should solve #644

Thanks @nemidiy for investigating this issue!

[kleini]

I tested this pull request on top of pull request #646. OTA partly seems to work. I see some firmware file transferred to my esp8266 devices, which is already a lot more compared to what was possible in the past, but after the firmware upload to the device I get 400 BAD_FIRMWARE.

I am using the firmware.bin file being created during pio run in the .pio/build/sonoff/firmware.bin file. Sometimes the firmware is not even transferred and the request is directly denied with the same error message.

[nemidiy]

I tested the @codefrey's PR on nodemcuv2 and nodemcu32 and works for both platforms.
The only difference is that for esp8266 the OTA updater script says that it failed (even though it did not) and for esp32 says it's successful, don't recall where but someone reported this same thing. Probably some bug in the python script.

@kleini when you applied the DNS commit as well and tested OTA, were us using dns1 and/or dns2 or did you just happen to apply the commit in #646 and tested without setting dns ? I am asking so that I can try it out myself. Thanks.

[kleini]

@nemidiy I tested on Saturday the exactly the following version https://github.com/kleini/homie-esp8266.git#35419c27ada5af4f1fc44fb19f339b355149599f, which is #646 and this pull request on top.

[nemidiy]

Hi @kleini I pulled develop-v3 and then merged this on top. I tested both setting up DNS and without DNS. OTA worked for me in both cases (tested on nodemcuv2). maybe you can put the details of your set up in the OTA ticket and I can try and help you out ? thanks.