zsh segfault bug from 5.9.1-1 onwards

I'm on Arch Linux, and a script of mine started to segfault after updating
from 5.9-6 to 5.9.1-1.

Minimal reproduction: run the following
```
(echo 1 | cat & (echo 2))
```

There's no segfault when:

   - Reverting to 5.9-6
   - removing either of the parenthesis
   - removing any command in the chain (replacing them can still segfault;
   my original code is quite different)