tools: Fix coverity issues

Summary:
tools/ldb_cmd.cc:
```
310  ignore_unknown_options_ = IsFlagPresent(flags, ARG_IGNORE_UNKNOWN_OPTIONS);

CID 1322798 (#1 of 1): Uninitialized pointer field (UNINIT_CTOR)
5. uninit_member: Non-static class member db_ttl_ is not initialized in this constructor nor in any functions that it calls.
311}
```
Closes https://github.com/facebook/rocksdb/pull/3122

Differential Revision: D6428576

Pulled By: sagar0

fbshipit-source-id: d77f04dd201f7f1d9f59ef88a215ee7ad7b934e9

tools/ldb_cmd.cc

@@ -279,6 +279,7 @@ LDBCommand::LDBCommand(const std::map<std::string, std::string>& options,
                        const std::vector<std::string>& flags, bool is_read_only,
                        const std::vector<std::string>& valid_cmd_line_options)
     : db_(nullptr),
+      db_ttl_(nullptr),
       is_read_only_(is_read_only),
       is_key_hex_(false),
       is_value_hex_(false),