In some cases (when there is an evaluated curve), the conversion code
would try to free the evaluated data-block twice, because freeing the
object would free it from `data_eval` and then the data-block was freed
again explicitly. Now check if the data-block is stored in `data_eval`
before freeing `object.data` manually. This is another area that's made
more complex by the fact that we change the meaning of `object.data`
for evaluated objects. The solution is more complicated than it should
be, but it works whether or not an evaluated mesh or curve exists.
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