Rewrite the issue-10734 rpass file

Stop relying on a malloc error to indicate failure and instead use an
explicit check. Also ensure that the value is dropped at the correct
time (e.g. that the if statement is translated into `{ expr }` instead
of `expr`).
auto merge of #10748 : kballard/rust/issue-10734-rpass, r=alexcrichton

Stop relying on a malloc error to indicate failure and instead use an
explicit check. Also ensure that the value is dropped at the correct
time (e.g. that the if statement is translated into `{ expr }` instead
of `expr`).