Fix an unnecessary use of `cast::transmute`

Wherever possible, more specialized variants of said functions should be used,
such as in this case `cast::transmute_mmut_unsafe`.
auto merge of #13343 : tbu-/rust/pr_smallfix, r=pcwalton

Fix an unnecessary use of `cast::transmute`

Wherever possible, more specialized variants of said functions should be used,
such as in this case `cast::transmute_mmut_unsafe`.