bugfix(object): Check whether a bike still has a rider before sniping it to prevent crashing the game

[Caball009]

Brought to my attention by @MrS-ibra

This PR fixes a crash caused by accessing the first element of an empty list. This bug happens when you use Jarmen Kell to snipe an empty bike. Normally, empty (enemy) bikes are immediately destroyed, but disabled bikes can be empty for as long as they're disabled.

Bug reproduction:

1. Disable an enemy bike, for instance with Black Lotus. Keep the bike disabled for all steps.
2. Dismount the rider from the disabled bike (or use Jarmen Kell to snipe the rider from the bike).
3. Use Jarmen Kell to snipe the now empty bike.
4. The game crashes here:
   

   GeneralsGameCode/GeneralsMD/Code/GameEngine/Source/GameLogic/Object/Body/ActiveBody.cpp

   Lines 408 to 409 in 87de0a0

   	//Removing the rider will scuttle the bike.
   	Object *rider = *(contain->getContainedItemsList()->begin());

PS: It probably shouldn't be possible to dismount from a disabled bike, but that's outside the scope of this PR.

[Caball009]

This could be fixed differently if we decide that empty (enemy) bikes should be scuttled immediately even when they're disabled.

[xezon]

Needs to be replicated to Generals.

Nevermind, Generals does not have bikes.