`data_overflow.rs` example/test is failing

[jonas-schievink]

Or rather, it builds successfully. The reason is that these ! do not work with errexit, so a successful build has no consequences:

https://github.com/rust-embedded/cortex-m-rt/blob/5ac023dd9261d67aafbeab018241b4bcc4742152/ci/script.sh#L44-L48

Caught by shellcheck SC2251.

[newAM]

I'm triaging Cortex-M issues, I think this one is fixed as of #505, please re-open if not!

[newAM]

Actually I didn't fix it in #505, have #542 open to fix.