Avoid using sizes on non-fixed-width types to derive protocol constants.

Thanks to awemany for pointing this out.

src/validation.cpp

@@ -20,6 +20,7 @@
 #include "init.h"
 #include "policy/fees.h"
 #include "policy/policy.h"
+#include "policy/rbf.h"
 #include "pow.h"
 #include "primitives/block.h"
 #include "primitives/transaction.h"
@@ -488,9 +489,9 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
             if (!setConflicts.count(ptxConflicting->GetHash()))
             {
                 // Allow opt-out of transaction replacement by setting
-                // nSequence >= maxint-1 on all inputs.
+                // nSequence > MAX_BIP125_RBF_SEQUENCE (SEQUENCE_FINAL-2) on all inputs.
                 //
-                // maxint-1 is picked to still allow use of nLockTime by
+                // SEQUENCE_FINAL-1 is picked to still allow use of nLockTime by
                 // non-replaceable transactions. All inputs rather than just one
                 // is for the sake of multi-party protocols, where we don't
                 // want a single party to be able to disable replacement.
@@ -504,7 +505,7 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
                 {
                     for (const CTxIn &_txin : ptxConflicting->vin)
                     {
-                        if (_txin.nSequence < std::numeric_limits<unsigned int>::max()-1)
+                        if (_txin.nSequence <= MAX_BIP125_RBF_SEQUENCE)
                         {
                             fReplacementOptOut = false;
                             break;

src/wallet/wallet.cpp

@@ -2723,7 +2723,7 @@ bool CWallet::CreateTransaction(const std::vector<CRecipient>& vecSend, CWalletT
                 // to avoid conflicting with other possible uses of nSequence,
                 // and in the spirit of "smallest possible change from prior
                 // behavior."
-                const uint32_t nSequence = coin_control.signalRbf ? MAX_BIP125_RBF_SEQUENCE : (std::numeric_limits<unsigned int>::max() - 1);
+                const uint32_t nSequence = coin_control.signalRbf ? MAX_BIP125_RBF_SEQUENCE : (CTxIn::SEQUENCE_FINAL - 1);
                 for (const auto& coin : setCoins)
                     txNew.vin.push_back(CTxIn(coin.outpoint,CScript(),
                                               nSequence));