Fix unfullfilled callbacks when gatt is null

[matt-42]

This fixes issue #1291 and a null exception.

[marcosinigaglia]

Hi @matt-42 , are you sure that is the correct fix? Have you test it?

[matt-42]

Yes, I've tested it and my connection problems got solved, and I did not have null pointer exceptions. gatt is checked almost everywhere. I'll add more check just in case.
There may be more problems though, since commandQueue.clear(); is called in other places without properly calling all the callbacks with an error message.

[matt-42]

I've changed a bit my PR so the callbacks get cleared as soon as we got the gatt is null error, also added error checking in onServicesDiscovered.

[marcosinigaglia]

Thanks @matt-42 , let me know when you think is ready to merge.

[matt-42]

ok :). I'll tell you when I'm done doing fixes.

[matt-42]

I'm done with my work. I did not have issues of hanging promises anymore and I did not break anything in my app. Feel free to merge it when you think it is the right time to do so :)

[vhakulinen]

After trying to fix similar issue in #1321 for v11, I came to realization that there is still at least one way for callbacks to say floating / unfulfilled: when a read call (or any other enqueued "command") is issued to a disconnected peripheral, nextCommand will notice that gatt is null and invoke errors to all callbacks. The issue here is that the errorAndClearAllCallbacks is called before the latest callback is added to the callback array (readCallbacks in case of a read).

EDIT: I guess that would be fixed by #1305