Improve XRP_t support for Number

- Use the same type of validation as for double

src/test/jtx/amount.h

@@ -295,10 +295,19 @@ struct XRP_t
         return {TOut{v} * dropsPerXRP};
     }
 
+    /** Returns an amount of XRP as PrettyAmount,
+        which is trivially convertable to STAmount
+
+        @param v The Number of XRP (not drops). May be fractional.
+    */
     PrettyAmount
     operator()(Number v) const
     {
-        return static_cast<int64_t>(v * dropsPerXRP);
+        auto const c = dropsPerXRP.drops();
+        auto const d = std::int64_t(v * c);
+        if (Number(d) / c != v)
+            Throw<std::domain_error>("unrepresentable");
+        return {d};
     }
 
     PrettyAmount