fix(gateway_balances): handle overflow exception: (#4355)

* Prevent internal error by catching overflow exception in `gateway_balances`.
* Treat `gateway_balances` obligations overflow as max (largest valid) `STAmount`.
  * Note that very large sums of STAmount are approximations regardless.

---------

Co-authored-by: Scott Schurr <scott@ripple.com>

src/ripple/rpc/handlers/GatewayBalances.cpp

@@ -184,7 +184,23 @@ doGatewayBalances(RPC::JsonContext& context)
                         bal = -rs->getBalance();
                     }
                     else
-                        bal -= rs->getBalance();
+                    {
+                        try
+                        {
+                            bal -= rs->getBalance();
+                        }
+                        catch (std::runtime_error const&)
+                        {
+                            // Presumably the exception was caused by overflow.
+                            // On overflow return the largest valid STAmount.
+                            // Very large sums of STAmount are approximations
+                            // anyway.
+                            bal = STAmount(
+                                bal.issue(),
+                                STAmount::cMaxValue,
+                                STAmount::cMaxOffset);
+                        }
+                    }
                 }
             });
     }