TestHoveringTank crashes if acceleration is applied during initial descent  #1126

The TestHoveringTank sample application starts with the tank gently descending until its invisible wheels touch down. The app will crash with a stream of execution exceptions and assertion failures if you press the "W" (accelerate) key during this period.

I think the bug is in PhysicsHoverControl. Initially, the tank's horizontal velocity is 0. When the "W" key is pressed, the accelerationValue > 0 branch of prePhysicsTick() is executed. Projecting dir onto a zero vector results in a NaN vector, which is eventually applied to the vehicle as a force.