Vim has heap-use-after-free at /src/charset.c:1770:12 in skipwhite

Vim is a UNIX editor that, prior to version 9.0.2121, has a heap-use-after-free vulnerability. When executing a `:s` command for the very first time and using a sub-replace-special atom inside the substitution part, it is possible that the recursive `:s` call causes free-ing of memory which may later then be accessed by the initial `:s` command. The user must intentionally execute the payload and the whole process is a bit tricky to do since it seems to work only reliably for the very first :s command. It may also cause a crash of Vim. Version 9.0.2121 contains a fix for this issue.