Fix sst_dump for PlainTable

Summary: sst_dump now doesn't work well for PlainTable. Not sure when it started, but this should fix it.

Test Plan: Run sst_dump against a file that used to fail.

Reviewers: yhchiang, haobo, igor

Reviewed By: igor

Subscribers: dhruba, ljin, leveldb

Differential Revision: https://reviews.facebook.net/D19023