Tiny fix to db_bench for make release.

Summary:
In release, "found variable assigned but not used anywhere". Changed it to work with
assert. Someone accept this :).

Test Plan: make release -j 32

Reviewers: haobo, dhruba, emayanke

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12309

db/db_bench.cc

@@ -1487,7 +1487,9 @@ class Benchmark {
 
         // We should not find any key since the key we try to get has a
         // different suffix
-        assert(!found);
+        if (found) {
+          assert(false);
+        }
 
         thread->stats.FinishedSingleOp(db_);
         keys_left -= num_keys;