Expanding a parameter with a non-ASCII name panics

[magicant]

Reproduced with commit ae888a0.

$ echo ${あ}

thread 'main' panicked at /home/magicant/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/annotate-snippets-0.11.5/src/renderer/display_list.rs:1213:45:
byte index 8 is not a char boundary; it is inside 'あ' (bytes 7..10) of `echo ${あ}
`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[magicant]

Fixed in #589