Regression from 427eed292d, in IDProperty case the 'ensured' PropertyRNA
pointer is some sort of empty template, and cannot be used for many
things.
Instead of calling `rna_ensure_property` twice for each call to
`RNA_property_animateable`, rather add an optional parameter to static
helper `rna_property_editable_do`, and call it directly from
`RNA_property_animateable`.