pht: use const reference for IndexEntry::TYPE

Prevents static initialization order issue

include/opendht/indexation/pht.h

@@ -266,7 +266,7 @@ private:
 
 using Value = std::pair<InfoHash, dht::Value::Id>;
 struct OPENDHT_PUBLIC IndexEntry : public dht::Value::Serializable<IndexEntry> {
-    static const ValueType TYPE;
+    static const ValueType& TYPE;
 
     virtual void unpackValue(const dht::Value& v) {
         Serializable<IndexEntry>::unpackValue(v);

src/indexation/pht.cpp

@@ -145,7 +145,7 @@ int Pht::Cache::lookup(const Prefix& p) {
     return pos;
 }
 
-const ValueType IndexEntry::TYPE = ValueType::USER_DATA;
+const ValueType& IndexEntry::TYPE = ValueType::USER_DATA;
 constexpr std::chrono::minutes Pht::Cache::NODE_EXPIRE_TIME;
 
 void Pht::lookupStep(Prefix p, std::shared_ptr<int> lo, std::shared_ptr<int> hi,