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

[via llvm-commits]

tanmaysachan wrote:

Null+0 is a UB, but even if we ignore that, the engine still crashes if string = null even with length = 0. I believe that's by design since the regex ^$ should match with length = 0 string, so the function should still execute (just not with a null string).

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