Skeleton._sortBones() crash for bones with parents from other skeletons

[EvgenyRodygin]

It is possible for a bone to have a parent bone from a skeleton different from the original one.
Currently, the method crashes when this is the case.
This simple check fixes it. Notice that "index" might be -1 when the parent is from another skeleton in the previous iteration.

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[deltakosh]

I'm deeply surprise you could get a bone with a different skeleton than the rest of the bones. This is clearly not supported by design

How do you end up with that?

Anyway this fix is great

[EvgenyRodygin]

I'm deeply surprise you could get a bone with a different skeleton than the rest of the bones. This is clearly not supported by design

How do you end up with that?

Anyway this fix is great

Yeah. Shouldn't be the case I know. You won't get something like this importing regular files I think (might be wrong though).
In our app we create a separate IK skeleton and the root bone of this IK skeleton is attached to the bone from the FK-skeleton. Might be not the best design though that what I encountered.