Improve an assertion in RandomGenerator::Generate() in db_bench.

Summary:
RandomGenerator::Generate() currently has an assertion len < data_.size().
However, it is actually fine to have len == data_.size().
This diff change the assertion to len <= data_.size().

Test Plan:
make db_bench
./db_bench

Reviewers: haobo, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19269