Don't double free embedded, unsized slices

Thanks to @eddyb for finding the bug.

Closes #16826 (I hope)
auto merge of #16838 : nick29581/rust/dst-bug-4, r=pnkfelix,nikomatsakis

Don't double free embedded, unsized slices.

Merge/rebase error from DST. Thanks to @eddyb for finding.

Closes #16826 (I hope)

r?