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.

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)

phpmainthread_test.go

@@ -96,7 +96,7 @@ func TestTransitionThreadsWhileDoingRequests(t *testing.T) {
 
 	var (
 		isDone atomic.Bool
-		wg sync.WaitGroup
+		wg     sync.WaitGroup
 	)
 
 	numThreads := 10

watcher.go

@@ -10,7 +10,7 @@ import (
 )
 
 type hotReloadOpt struct {
-	hotReload   []*watcher.PatternGroup
+	hotReload []*watcher.PatternGroup
 }
 
 var restartWorkers atomic.Bool