Avoiding undefined behaviour in lexers

[Neil Hodgson]

The recent fuzzing performed on Lexilla by Allen Yen showed some undefined behaviour caused by out of range reads of document text bytes. These are commonly a short distance outside the buffer and still within its containing object so are fairly mild.

However, projects are becoming more stringent in wanting to prevent any potentially dangerous actions. The security threat environment has grown worse over time and even 'benign' undefined behaviour can make auditing more difficult.

There are currently safe and unsafe variants of reading a document byte by index, SafeGetCharAt and operator[]. I am proposing to make operator[] as safe as  SafeGetCharAt.

https://github.com/ScintillaOrg/lexilla/issues/392

Since this change will affect many call sites, it will be done after the next Lexilla release 5.5.5 so that it is well tested before distribution.

Neil