Fix SIGFAULT when running sst_dump on v2.6 db

Summary: Fix the sigfault when running sst_dump on v2.6 db.

Test Plan:
    git checkout bba6595b1f3f42cf79bb21c2d5b981ede1cc0063
    make clean
    make db_bench
    ./db_bench --db=/tmp/some/db --benchmarks=fillseq
    arc patch D18039
    make clean
    make sst_dump
    ./sst_dump --file=/tmp/some/db --command=check

Reviewers: igor, haobo, sdong

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18039