Fix errors in clang-analyzer builds

Fix errors in clang-analyzer builds

Summary:
Fix build error in db_iter.cc when running clang-analyzer.
```
  CC       db/db_iter.o
db/db_iter.cc:938:21: error: no matching constructor for initialization of 'rocksdb::ParsedInternalKey'
  ParsedInternalKey ikey(Slice(), 0, 0);
                    ^    ~~~~~~~~~~~~~
./db/dbformat.h:84:3: note: candidate constructor not viable: no known conversion from 'int' to 'rocksdb::ValueType' for 3rd argument
  ParsedInternalKey(const Slice& u, const SequenceNumber& seq, ValueType t)
  ^
./db/dbformat.h:78:8: note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 3 were provided
struct ParsedInternalKey {
       ^
./db/dbformat.h:78:8: note: candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 3 were provided
./db/dbformat.h:83:3: note: candidate constructor not viable: requires 0 arguments, but 3 were provided
  ParsedInternalKey() { }  // Intentionally left uninitialized (for speed)
  ^
1 error generated.
```
Closes https://github.com/facebook/rocksdb/pull/2354

Differential Revision: D5115751

Pulled By: sagar0

fbshipit-source-id: b0e386d4e935e4725b07761c3ca5f7a8cbde3692