Correct handling of LoanBroker.DebtMaximum zero values

- Addresses FIND-003 from audit.
- Behavior changed to treat a DebtMaximum value of 0 to mean "no limit",
  as defined in spec.
- No need to add unit tests, because the previous commit covers 0, and
  tests already exist for non-zero limits.

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

@@ -243,7 +243,8 @@ LoanSet::preclaim(PreclaimContext const& ctx)
         return tecINSUFFICIENT_FUNDS;
     }
     auto const newDebtTotal = brokerSle->at(sfDebtTotal) + principalRequested;
-    if (brokerSle->at(sfDebtMaximum) < newDebtTotal)
+    if (auto const debtMaximum = brokerSle->at(sfDebtMaximum);
+        debtMaximum != 0 && debtMaximum < newDebtTotal)
     {
         JLOG(ctx.j.warn())
             << "Loan would exceed the maximum debt limit of the LoanBroker.";