Prevent subtract with overflow for word "ion"

[jobarr-amzn]

I recently ran into this error through an mdbook build, also reported by someone else here.

Luckily it was pretty straightforward to find and fix :) Thanks for maintaining this repository!

See also:

• prevent 'subtract with overflow' with 4 char word minhnhdo/rust-stem#9
• Release 2 notes from the reference implementation:

   Release 2: 11 Apr 2013
       fixes a bug noted by Matt Patenaude <[email protected]>,

       case 'o': if (ends("\03" "ion") && (b[j] == 's' || b[j] == 't')) break;
           ==>
       case 'o': if (ends("\03" "ion") && j >= k0 && (b[j] == 's' || b[j] == 't')) break;

       to avoid accessing b[k0-1] when the word in b is "ion".

[jobarr-amzn]

Note this does not include the patch from the reference implementation Release 3- I spent a few minutes looking into that and didn't grok the Rust port well enough to get the exact same behavior as the reference implementation. It would be easy to just copy the step2 and step4 subtract overflow guards from rust-stem#9, but that still fails if you add an "ied" => "ied" test case- I'm happy to leave that to posterity. @thecodrr ?

[mattico]

Thanks! I'll look into the other changes when I can. PRs welcome!