fix: prevent segfault on early shutdown. (#2120)

Fixes #2114

This early
[Shutdown](11160fb7b3/frankenphp.go (L281))
introduced in #2031 segfaults instead of returning an error since
threads have not started yet.

phpmainthread.go

@@ -79,6 +79,9 @@ func initPHPThreads(numThreads int, numMaxThreads int, phpIni map[string]string)
 }
 
 func drainPHPThreads() {
+	if mainThread == nil {
+		return // mainThread was never initialized
+	}
 	doneWG := sync.WaitGroup{}
 	doneWG.Add(len(phpThreads))
 	mainThread.state.Set(state.ShuttingDown)