insert_slice_clone can double drop if Clone panics.

Affected versions of this crate used ptr::copy when inserting into the middle
of a Vec. When ownership was temporarily duplicated during this copy, it calls
the clone method of a user provided element.

This issue can result in an element being double-freed if the clone call panics.

Commit 20cb73d fixed this issue by adding a set_len(0) call before
operating on the vector to avoid dropping the elements during a panic.