Fix panic on scanning indentation of arbitrary lines

[yuja]

thread 'main' panicked at cargo-insta/src/inline.rs:130:22:
byte index 32 is out of bounds of `    insta::assert_snapshot!(`

[ilyagr]

I'm a bit confused about this PR. It conflicts with #722 and my naive attempt to resolve the conflict resulted in broken tests.

Perhaps it should be a draft until #722 is merged?

[yuja]

I can rebase this or #722 as needed. They're different patches and I think this bug is more important than #722 because it's a regression.

[max-sixty]

Thanks a lot @yuja !

If you want to add a changelog + attribution later, that would be great! No obligation though

[yuja]

The bug doesn't exist in the released version, so I don't think changelog entry is needed. Thanks for reviewing.