Fix refering freed memory in earlier commit.

Summary: Fix refering freed memory in earlier commit by https://reviews.facebook.net/D11181

Test Plan: make check

Reviewers: haobo, sheki

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11193

db/dbformat.cc

@@ -44,8 +44,7 @@ std::string InternalKey::DebugString(bool hex) const {
 }
 
 const char* InternalKeyComparator::Name() const {
-  return ("leveldb.InternalKeyComparator:" +
-          std::string(user_comparator_->Name())).c_str();
+  return name_.c_str();
 }
 
 int InternalKeyComparator::Compare(const Slice& akey, const Slice& bkey) const {

db/dbformat.h

@@ -86,8 +86,13 @@ inline ValueType ExtractValueType(const Slice& internal_key) {
 class InternalKeyComparator : public Comparator {
  private:
   const Comparator* user_comparator_;
+  std::string name_;
  public:
-  explicit InternalKeyComparator(const Comparator* c) : user_comparator_(c) { }
+  explicit InternalKeyComparator(const Comparator* c) : user_comparator_(c),
+    name_("leveldb.InternalKeyComparator:" +
+          std::string(user_comparator_->Name())) {
+  }
+
   virtual const char* Name() const;
   virtual int Compare(const Slice& a, const Slice& b) const;
   virtual void FindShortestSeparator(