Use const in COutPoint class

Author: hebasto

src/primitives/transaction.h

@@ -21,7 +21,9 @@ class COutPoint
     uint256 hash;
     uint32_t n;
 
-    COutPoint(): n((uint32_t) -1) { }
+    static constexpr uint32_t NULL_INDEX = std::numeric_limits<uint32_t>::max();
+
+    COutPoint(): n(NULL_INDEX) { }
     COutPoint(const uint256& hashIn, uint32_t nIn): hash(hashIn), n(nIn) { }
 
     ADD_SERIALIZE_METHODS;
@@ -32,8 +34,8 @@ class COutPoint
         READWRITE(n);
     }
 
-    void SetNull() { hash.SetNull(); n = (uint32_t) -1; }
-    bool IsNull() const { return (hash.IsNull() && n == (uint32_t) -1); }
+    void SetNull() { hash.SetNull(); n = NULL_INDEX; }
+    bool IsNull() const { return (hash.IsNull() && n == NULL_INDEX); }
 
     friend bool operator<(const COutPoint& a, const COutPoint& b)
     {