Investigate tarpaulin segfault

[brson]

In the w3f m1 evaluation, @lucasvanmaul discovered that running cargo tarpaulin on the project causes a segfault.

https://github.com/w3f/Grant-Milestone-Delivery/blob/master/evaluations/wasm-opt-rust_1_lucasvanmol.md

[brson]

Can't solve this, but we'll file an issue against tarpaulin.