Initialize output parameters

If the function returns true, it should
set all output paremeters, similar to Output::preflightElement, or we
have UB on code like:

```
void *SaveInfo;
if (io.preflightFlowElement(i, SaveInfo))
  io.postflightFlowElement(SaveInfo);
```

It's going to be detected by msan with:
-Xclang -enable-noundef-analysis -mllvm -msan-eager-checks=1

Differential Revision: https://reviews.llvm.org/D116826