Fix memory leak in KeyMayExist test part of db_test

Summary: NewBloomFilterPolicy call requires Delete to be called later on

Test Plan: make; valgrind ./db_test

Reviewers: haobo, dhruba, vamsi

Differential Revision: https://reviews.facebook.net/D11667

db/db_test.cc

@@ -800,6 +800,8 @@ TEST(DBTest, KeyMayExist) {
 
     ASSERT_OK(db_->Delete(WriteOptions(), "c"));
     ASSERT_TRUE(!db_->KeyMayExist("c"));
+
+    delete options.filter_policy;
   } while (ChangeOptions());
 }