fix: Use ".value()" instead of "->" when with STObject::Proxy objects

- Turns out that "Proxy::operator->" is not a safe substitute for
  "Proxy::value()." if the field is not required. The implementation
  is different such that "operator->" will return a null ptr if the
  field is not present. This includes default fields with a value of
  zero!

include/xrpl/protocol/STObject.h

@@ -482,6 +482,8 @@ public:
     value_type
     operator*() const;
 
+    /// Do not use operator->() unless the field is required, or you've checked
+    /// that it's set.
     T const*
     operator->() const;
 
@@ -739,6 +741,8 @@ STObject::Proxy<T>::operator*() const -> value_type
     return this->value();
 }
 
+/// Do not use operator->() unless the field is required, or you've checked that
+/// it's set.
 template <class T>
 T const*
 STObject::Proxy<T>::operator->() const

src/xrpld/app/tx/detail/LoanManage.cpp

@@ -175,7 +175,7 @@ LoanManage::defaultLoan(
         // The vault may be at a different scale than the loan. Reduce rounding
         // errors during the accounting by rounding some of the values to that
         // scale.
-        auto const vaultScale = vaultTotalProxy->value().exponent();
+        auto const vaultScale = vaultTotalProxy.value().exponent();
 
         if (vaultTotalProxy < vaultDefaultAmount)
         {
@@ -197,13 +197,12 @@ LoanManage::defaultLoan(
             auto const difference = vaultAvailableProxy - vaultTotalProxy;
             JLOG(j.debug())
                 << "Vault assets available: " << *vaultAvailableProxy << "("
-                << vaultAvailableProxy->value().exponent()
+                << vaultAvailableProxy.value().exponent()
                 << "), Total: " << *vaultTotalProxy << "("
-                << vaultTotalProxy->value().exponent()
+                << vaultTotalProxy.value().exponent()
                 << "), Difference: " << difference << "("
                 << difference.exponent() << ")";
-            if (vaultAvailableProxy->value().exponent() -
-                    difference.exponent() >
+            if (vaultAvailableProxy.value().exponent() - difference.exponent() >
                 13)
             {
                 // If the difference is dust, bring the total up to match

src/xrpld/app/tx/detail/LoanPay.cpp

@@ -351,7 +351,7 @@ LoanPay::doApply()
 
     // The vault may be at a different scale than the loan. Reduce rounding
     // errors during the payment by rounding some of the values to that scale.
-    auto const vaultScale = assetsTotalProxy->value().exponent();
+    auto const vaultScale = assetsTotalProxy.value().exponent();
 
     auto const totalPaidToVaultRaw =
         paymentParts->principalPaid + paymentParts->interestPaid;

src/xrpld/app/tx/detail/LoanSet.cpp

@@ -420,7 +420,7 @@ LoanSet::doApply()
 
     auto vaultAvailableProxy = vaultSle->at(sfAssetsAvailable);
     auto vaultTotalProxy = vaultSle->at(sfAssetsTotal);
-    auto const vaultScale = vaultTotalProxy->value().exponent();
+    auto const vaultScale = vaultTotalProxy.value().exponent();
     if (vaultAvailableProxy < principalRequested)
     {
         JLOG(j_.warn())
@@ -513,7 +513,7 @@ LoanSet::doApply()
         auto const ownerCount = borrowerSle->at(sfOwnerCount);
         auto const balance = account_ == borrower
             ? mPriorBalance
-            : borrowerSle->at(sfBalance)->xrp();
+            : borrowerSle->at(sfBalance).value().xrp();
         if (balance < view.fees().accountReserve(ownerCount))
             return tecINSUFFICIENT_RESERVE;
     }

src/xrpld/app/tx/detail/VaultDeposit.cpp

@@ -273,7 +273,8 @@ VaultDeposit::doApply()
     }
     assetsTotalProxy += assetsDeposited;
     assetsAvailableProxy += assetsDeposited;
-    if (!assetsTotalProxy->valid() || !assetsAvailableProxy->valid())
+    if (!assetsTotalProxy.value().valid() ||
+        !assetsAvailableProxy.value().valid())
         return tecLIMIT_EXCEEDED;
     view().update(vault);