More methods from `IonReader` could throw an unexpected `AssertionError`

[arthurscchan]

Following #417, it is discovered that more methods from IonReader could throw an unexpected AssertionError. From #417, it is known that IonReader::stringValue() which is served by an Amazon implementation of IonReaderTextSystemX will throw AssertionError when the resolved symbol id is 0 or negative. Although it has been caught by the direct call from the IonParser::getText() method, it is found that the call to IonReader::next() from IonParser.nextToken() will also invoke IonReader::stringValue() in some cases and cause unexpected AssertionError.

    @Override
    public JsonToken nextToken() throws IOException
    {
        // special case: if we return field name, we know value type, return it:
        if (_currToken == JsonToken.FIELD_NAME) {
            return (_currToken = _valueToken);
        }
        // also, when starting array/object, need to create new context
        if (_currToken == JsonToken.START_OBJECT) {
            _parsingContext = _parsingContext.createChildObjectContext(-1, -1);
            _reader.stepIn();
        } else if (_currToken == JsonToken.START_ARRAY) {
            _parsingContext = _parsingContext.createChildArrayContext(-1, -1);
            _reader.stepIn();
        }

        // any more tokens in this scope?
        IonType type = null;
        try {
            type = _reader.next();
...

The fix is similar to #418, to catch and wrap the AssertionError.

We found this issue by OSS-Fuzz and it is reported in https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=65273.