stop calling `exchange_free` on 0-size types

A solid step towards fixing #11998. Eventually, the size may always be
passed to `exchange_free` but this will not be required to fix the bug.
auto merge of #12045 : thestinger/rust/glue, r=pcwalton

A solid step towards fixing #11998. Eventually, the size may always be
passed to `exchange_free` but this will not be required to fix the bug.