Add default constructor to RecordIOReader::Iterator
Add default constructor to RecordIOReader::Iterator

Summary: For this iterator to satisfy the `input_or_output_iterator` concept in range-v3, it must also satisfy the `semiregular` concept, which requires a default constructor.

Reviewed By: yfeldblum

Differential Revision: D18689298

fbshipit-source-id: a38cde0041ceba1aac79128d428df05ee0f00ab4

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