Subject: crash editing bone parent after duplicate & delete

make skeleton layer
export 1
in layers: copy, paste, delete
clear history
edit bone1's parent

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When we try to edit bone1's parent, the first thing it does it tries
to find out which bones are being affected by the widget we're
editing.  To do this it walks up the parent_set tree from the
valuenode we're editing.

The parent sub-param of bone1 that we're trying to edit has 1 parent: bone1
bone1 has 3 parents:
  the still-alive static_list
  the copy of static_list on the clipboard
  the pasted static_list (which has been deleted already)
