JSONObject and JSONArray initial flag for tokener may be flawed

[stleary]

From #927:

The boolean isInitial = x.getPrevious() == 0; check can behave in an unexpected way, for example:
JSONParserConfiguration strict = new JSONParserConfiguration().withStrictMode();
JSONTokener tok = new JSONTokener("[]xxx");
// Calling next+back changes "previous"; without these calls the check is performed:
tok.next();
tok.back();
JSONArray a1 = new JSONArray(tok, strict); // No error here, but should be because of extra chars?

This is a far corner case I think, but should be fixed if practical.
Is there a way to capture this in the JSONTokener object?