[BUG] Use arith_uint256 for old modifier's block sorting

src/legacy/stakemodifier.cpp

@@ -130,6 +130,16 @@ bool GetOldStakeModifier(CStakeInput* stake, uint64_t& nStakeModifier)
     return true;
 }
 
+// sort blocks by timestamp, soliving tie with hash (taken as arith_uint)
+static bool sortedByTimestamp(const std::pair<uint64_t, uint256>& a,
+                              const std::pair<uint64_t, uint256>& b)
+{
+    if (a.first == b.first) {
+        return UintToArith256(a.second) < UintToArith256(b.second);
+    }
+    return a.first < b.first;
+}
+
 // Stake Modifier (hash modifier of proof-of-stake):
 // The purpose of stake modifier is to prevent a txout (coin) owner from
 // computing future proof-of-stake generated by this txout at the time
@@ -183,7 +193,7 @@ bool ComputeNextStakeModifier(const CBlockIndex* pindexPrev, uint64_t& nStakeMod
     }
 
     std::reverse(vSortedByTimestamp.begin(), vSortedByTimestamp.end());
-    std::sort(vSortedByTimestamp.begin(), vSortedByTimestamp.end());
+    std::sort(vSortedByTimestamp.begin(), vSortedByTimestamp.end(), sortedByTimestamp);
 
     // Select 64 blocks from candidate blocks to generate stake modifier
     uint64_t nStakeModifierNew = 0;