[llvm] [Support] Fix buffer overflow in regcomp (PR #76681)

[via llvm-commits]

DavidKorczynski wrote:

I'm not entirely sure about what to do with the formatting -- the current PR uses tabs as the rest of `regcomp.c` does that. However, I'm being asked by the formatter to change my additions to spaces, although this will be inconsistent with the rest of `regcomp.c`.

What to do?

https://github.com/llvm/llvm-project/pull/76681