Add Class Variable Declaration for _backoff_at_millis

[kevinlutzer]

Setup

• System: Macbook Air; OSX 0.14.3
• Arduino: 1.8.12
• Library Version: 1.1.9

Problem

When I try and compile the Esp8266-lwmqtt code with the library installed via the .zip method I get an error:

error: '_backoff_at_millis' was not declared in this scope
       _backoff_at_millis = millis() + this->__backoff__;

This is because on master the _backoff_at_millis variable is not defined for the CloudIoTCoreMqtt class. I can't replicate when I install the same library version using the Arduino Library Manager, because that version doesn't contain that variable.

Solution

Define _backoff_at_millis in the CloudIoTCoreMqtt class.

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[kevinlutzer]

@googlebot I signed it

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[DanielGarc]

I think the variable declaration should be an unsigned long since it's compared with millis() returns an unsigned long

Also it should be initialized 0

[Thanasist2]

CloudIoTCoreMqtt.h line 31 add -->unsigned long _backoff_at_millis = 0

esp8266/Arduino#6811
#define _stackSize (6748/4)
mine is" BSSL stack: 6132"

[gguuss]

LGTM