When parsing negative exponents, the current implementation takes
precautions for |exp| to not underflow int.
But that is not sufficient: later on [1], |exp + expFrac| is also
stored to an int - so we must ensure that the sum stays within int
representable values.
Update the exp clamping logic to take expFrac into account.
[1] https://github.com/Tencent/rapidjson/blob/master/include/rapidjson/reader.h#L1690
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