[llvm] [FIX] Fix undefined-behaviour in regex engine. (PR #73071)

[Nikita Popov via llvm-commits]

nikic wrote:

> Any chance of a test case? (Not sure, but I'd expect we have unit tests for the regex api that could be extended to cover this)

We do have some tests here: https://github.com/llvm/llvm-project/blob/main/llvm/unittests/Support/RegexTest.cpp

https://github.com/llvm/llvm-project/pull/73071