Small fixes for Loan merge

- Skip roundToScale work for 0 values.
- Expand a couple of test values to higher precision.

src/libxrpl/protocol/STAmount.cpp

@@ -1473,6 +1473,10 @@ roundToScale(
     if (value.integral())
         return value;
 
+    // Nothing to do for zero.
+    if (value == beast::zero)
+        return value;
+
     // If the value's exponent is greater than or equal to the scale, then
     // rounding will do nothing, and might even lose precision, so just return
     // the value.

src/test/app/Loan_test.cpp

@@ -2392,7 +2392,7 @@ protected:
                     interval * Number(12, -2) / secondsInYear;
                 BEAST_EXPECT(
                     periodicRate ==
-                    Number(2283105022831050, -21, Number::unchecked{}));
+                    Number(2283105022831050228ULL, -24, Number::normalized{}));
                 STAmount const principalOutstanding{
                     broker.asset, state.principalOutstanding};
                 STAmount const accruedInterest{
@@ -2624,7 +2624,7 @@ protected:
                     interval * Number(12, -2) / secondsInYear;
                 BEAST_EXPECT(
                     periodicRate ==
-                    Number(2283105022831050, -21, Number::unchecked{}));
+                    Number(2283105022831050228, -24, Number::normalized{}));
                 STAmount const roundedPeriodicPayment{
                     broker.asset,
                     roundPeriodicPayment(