Improve out-of-memory handling

Created by: firepick1

Benoit, I made a few tweaks in my branch to handle several out-of-memory edge cases that were blowing up my code. You might be interested in this: https://github.com/firepick1/ArduinoJson/commit/bc8c3d988e1f4a0fba3207ab25b5a086f2af36e8