Integrate VirtualBones crash fix into next release patch

Athomas reached out to me with a crash he was having when trying his 'Automatic Sync Marker AnimModifier' on a Skeleton that had Virtual Bones.

It was verified that this only happens when Virtual Bones are present.
He did package his plug-in for testing: [Link Removed]

To repro, add this AnimModifier on an animation. Run it, it works.
Add a virtual bone, add the modifier to another animation, run it, it crashes.

The fix for the crash is in CL #3478424 and I think we should integrate it in the next patch for the current release.

The problem is the Anim Modifier creates a smaller BoneContainer, with just the bones it needs. When this does not include the virtual bone, then the BoneContainer.VBCompactPoseData will contain INDEX_NONE bone indices. And in the AnimSequence::GetBonePose, it won't check for that and crash.

Although the more appropriate fix seemed to just not populate the VBCompactPoseData with invalid entries, rather than adding more checks to GetBonePose.