Fixed a memory leak issue in DBTest.UnremovableSingleDelete

Summary: Fixed a memory leak issue in DBTest.UnremovableSingleDelete

Test Plan: valgrind --error-exitcode=2 --leak-check=full ./db_test --gtest_filter="*UnremovableSingleDelete*"

Reviewers: sdong, anthony, IslamAbdelRahman, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D47583

db/db_test.cc

@@ -3691,6 +3691,7 @@ TEST_F(DBTest, UnremovableSingleDelete) {
 
     ASSERT_EQ("first", Get(1, "foo", snapshot));
     ASSERT_EQ("NOT_FOUND", Get(1, "foo"));
+    db_->ReleaseSnapshot(snapshot);
     // Skip HashCuckooRep as it does not support single delete.  FIFO and
     // universal compaction do not apply to the test case.  Skip MergePut
     // because single delete does not get removed when it encounters a merge.