Recalculate shop prices before giving money back on sold items.

This fixes a bug where one could get more than half the item price back. In
particularly, a level 2 Plasma Cooling Booster would sell for more than you
bought it for.

This fixes Debian bug #253380.

code/shop.cpp

@@ -710,6 +710,7 @@ void sell(int i)
 			break;
 	}
 
+	adjustShopPrices();
 	currentGame.cash += (shopItems[i].price / 2);
 }