fix: SIGSEGV with env vars

[Alliballibaba2]

This is another attempt at fixing #1269
I still couldn't reproduce the issue, but here is my best guess:

Some PHP extensions probably access environment variables when starting the main thread. In a past PR, we changed it so all environment is accessed via go

frankenphp/frankenphp.c

Line 777 in 0fc6ccc

frankenphp_getenv, /* getenv */

So we need to make sure the phpThreads slice is initialized before starting the main thread, so it can pin to the first phpThread (threadIndex on the main thread defaults to 0)

[Alliballibaba2]

I also changed it to explicitly check for a nil return on unsafe.StringData(s) (in case that' somehow the issue)

[dunglas]

LGTM

[dunglas]

Thanks!