add assert to silence clang analyzer and fix variable shadowing (#5140)

Summary:
This PR address two open issues:

1.  clang analyzer is paranoid about db_ being nullptr after DB::Open calls in the test.
See https://github.com/facebook/rocksdb/pull/5043#discussion_r271394579
Add an assert to keep clang happy
2. PR https://github.com/facebook/rocksdb/pull/5049 introduced a  variable shadowing:
```
db/db_iterator_test.cc: In constructor ‘rocksdb::DBIteratorWithReadCallbackTest_ReadCallback_Test::TestBody()::TestReadCallback::TestReadCallback(rocksdb::SequenceNumber)’:
db/db_iterator_test.cc:2484:9: error: declaration of ‘max_visible_seq’ shadows a member of 'this' [-Werror=shadow]
         : ReadCallback(max_visible_seq) {}
         ^
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5140

Differential Revision: D14735497

Pulled By: miasantreble

fbshipit-source-id: 3219ea75cf4ae04f64d889323f6779e84be98144

db/db_iterator_test.cc

@@ -2480,8 +2480,8 @@ class DBIteratorWithReadCallbackTest : public DBIteratorTest {};
 TEST_F(DBIteratorWithReadCallbackTest, ReadCallback) {
   class TestReadCallback : public ReadCallback {
    public:
-    explicit TestReadCallback(SequenceNumber max_visible_seq)
+    explicit TestReadCallback(SequenceNumber _max_visible_seq)
-        : ReadCallback(max_visible_seq) {}
+        : ReadCallback(_max_visible_seq) {}
 
     bool IsVisibleFullCheck(SequenceNumber seq) override {
       return seq <= max_visible_seq_;

db/deletefile_test.cc

@@ -71,7 +71,9 @@ class DeleteFileTest : public testing::Test {
     }
     db_ = nullptr;
     options_.create_if_missing = create;
-    return DB::Open(options_, dbname_, &db_);
+    Status s = DB::Open(options_, dbname_, &db_);
+    assert(db_);
+    return s;
   }
 
   void CloseDB() {