Infinite loop in UAnimSequence::ExtractRootMotion when sampling backwards

When sampling root motion backwards we can hit an infinite loop in ExtractRootMotion. This happens if the initial sample time + negative delta is off the end of the animation clip. In that case FAnimationRuntime::AdvanceTime() ends up clamping CurrentPosition so that it's the same as PreviousPosition meaning that our ActualDeltaMove is zero and we get stuck in the while loop.

1. Call ExtractRootMotion passing a StartTime off the end of the anim clip and a negative delta that doesn't take us to a sample point during the duration of the clip.
   1. eg. ThirdPersonIdle -> StartTime=5.0, DeltaTime=-1.0, AllowLooping=true
   2. This can be done via the following blueprint lib method:

       void UAnimationBlueprintLibrary::ExtractRootMotionFromSequence(const UAnimSequence* AnimationSequence, float StartTime, float DeltaTime, bool bAllowLooping)
      {
      	if (AnimationSequence)
      	{
      		AnimationSequence->ExtractRootMotion(StartTime, DeltaTime, bAllowLooping);
      	}
      }