Miri: fix determining size of an "extra function" allocation Fixes [a bug](https://github.com/rust-lang/miri/pull/862) introduced by https://github.com/rust-lang/rust/pull/62982. Best reviewed commit-by-commit. r? @oli-obk
Miri: fix determining size of an "extra function" allocation Fixes [a bug](https://github.com/rust-lang/miri/pull/862) introduced by https://github.com/rust-lang/rust/pull/62982. Best reviewed commit-by-commit. r? @oli-obk