Fix #9440: negative cargo payments not being handled right

Cargo payments were stored as unsigned integer, but cast to int64 during
application of inflation. However, then being multiplied with a uint64
making the result uint64. So in the end the payment that should have been
negative becomes hugely positive.

src/cargotype.h

@@ -61,7 +61,7 @@ struct CargoSpec {
 	uint8 rating_colour;
 	uint8 weight;                    ///< Weight of a single unit of this cargo type in 1/16 ton (62.5 kg).
 	uint16 multiplier;               ///< Capacity multiplier for vehicles. (8 fractional bits)
-	uint32 initial_payment;          ///< Initial payment rate before inflation is applied.
+	int32 initial_payment;           ///< Initial payment rate before inflation is applied.
 	uint8 transit_days[2];
 
 	bool is_freight;                 ///< Cargo type is considered to be freight (affects train freight multiplier).

src/economy.cpp

@@ -803,7 +803,7 @@ void RecomputePrices()
 
 	/* Setup cargo payment */
 	for (CargoSpec *cs : CargoSpec::Iterate()) {
-		cs->current_payment = ((int64)cs->initial_payment * _economy.inflation_payment) >> 16;
+		cs->current_payment = (cs->initial_payment * (int64)_economy.inflation_payment) >> 16;
 	}
 
 	SetWindowClassesDirty(WC_BUILD_VEHICLE);

src/economy_type.h

@@ -34,7 +34,7 @@ struct Economy {
 	uint32 industry_daily_change_counter; ///< Bits 31-16 are number of industry to be performed, 15-0 are fractional collected daily
 	uint32 industry_daily_increment;      ///< The value which will increment industry_daily_change_counter. Computed value. NOSAVE
 	uint64 inflation_prices;              ///< Cumulated inflation of prices since game start; 16 bit fractional part
-	uint64 inflation_payment;             ///< Cumulated inflation of cargo paypent since game start; 16 bit fractional part
+	uint64 inflation_payment;             ///< Cumulated inflation of cargo payment since game start; 16 bit fractional part
 
 	/* Old stuff for savegame conversion only */
 	Money old_max_loan_unround;           ///< Old: Unrounded max loan