fix: prevent segfault on early shutdown. (#2120)
Fixes #2114
This early
[Shutdown](https://github.com/php/frankenphp/blob/11160fb7b31171d706cf9933abb9102dbb1cdb3c/frankenphp.go#L281)
introduced in #2031 segfaults instead of returning an error since
threads have not started yet.
