Fix boundary check in script generated maps

[aco4]

The documentation for runMap_setMapData says:

//MAP-- `droids` is an array of droid data objects. Example:
//MAP--    {name: "ConstructionDroid", position: [x, y], direction: gameRand(0x10000), player: 1}
//MAP-- `features` is an array of feature data objects. Example:
//MAP--    {name: "Tree1", position: [x, y], direction: gameRand(0x10000)}

However, the code

gameRand(0x10000)

which generates a random value between 0 and 65535 (inclusive), would actually throw an error if it picks 65535. I fixed this