[compiler-rt] Work around a C language bug in libFuzzer (PR #96775)

[Dan McArdle via llvm-commits]

https://github.com/dmcardle approved this pull request.

Not an expert here, but these changes LGTM.

I was a little suspicious of the `memcpy` in `MutateAndTestOne()`, but it turns out that `ChooseUnitToMutate()` asserts that the returned unit is not empty.

https://github.com/llvm/llvm-project/pull/96775