ZgatewayBT::launchBTDiscovery theeng properties json deserialization fails with NoMemory on ESP32

[ssalonen]

ZgatewayBT::launchBTDiscovery theeng properties json deserialization fails with NoMemory on ESP32

[1technophile]

@ssalonen thanks for the analysis.
I would suggest the fix number 2, and increasing to 816 for ESP32, the other boards with lower ram will not be impacted as they have a separate definition for JSON_MSG_BUFFER.
Also Arduino boards support will be removed soon.

[ssalonen]

@1technophile I can happily provide a fix for this one, but let me know if there's something else to try out that would cause a crash

[ssalonen]

I also note here that ruuvi is having longest properties (json props json string), at 513 bytes. Second largest is 409 bytes (device index 34, thermobeacon). Of course the StaticJsonDocument needs some more for the overhead

This gives a good indication that there are no other tricky devices

[ssalonen]

I will file a PR bit later.

Will also include error handling

[ssalonen]

The regression might have happened in recent PR where props have become larger, eg

theengs/decoder#359
Or
theengs/decoder#452

[ssalonen]

See PR #1906 for the fix.

I also propose we log any deserialization errors, see #1907 (the rest of the function body still would get executed). In #1907 I did not tweak the implementation any further to avoid access invalid pointers (like what happened here) - I would expect a crash with ESP8266.