Hotfix for CVE-2021-43518. Closes #2981. Adding all the datafile validation (#2931) ended up being a rather large patch and it still needs some work. This hotfix just addresses the exploitable stack buffer overflow write. Fixing the other issues individually would be pointless, as there are at least 50-100 more distinct maps that can crash the client or cause ASAN/UBSAN errors.

Reserve one additional byte to ensure null termination in linereader and prevent OOB access when reading a very long line.

Fixes handling of text files encoded with UTF-8 BOM. This changes io_open so the UTF-8 BOM signature is transparently skipped if the flag IOFLAG_SKIP_BOM is set. It is not necessary to skip the BOM when reading json files, as the json parser already does this on its own.