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

db/db_bench.cc

@@ -584,9 +584,9 @@ class RandomGenerator {
   }
 
   Slice Generate(unsigned int len) {
+    assert(len <= data_.size());
     if (pos_ + len > data_.size()) {
       pos_ = 0;
-      assert(len < data_.size());
     }
     pos_ += len;
     return Slice(data_.data() + pos_ - len, len);