Restore mutex for MP4

cmd/mp4/mp4.go

@@ -4,6 +4,7 @@ import (
 	"net/http"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/AlexxIT/go2rtc/cmd/api"
@@ -108,9 +109,14 @@ func handlerMP4(w http.ResponseWriter, r *http.Request) {
 		Medias:     core.ParseQuery(r.URL.Query()),
 	}
 
+	var mu sync.Mutex
 	cons.Listen(func(msg any) {
 		if data, ok := msg.([]byte); ok {
-			if _, err := w.Write(data); err != nil && exit != nil {
+			mu.Lock()
+			_, err := w.Write(data)
+			mu.Unlock()
+
+			if err != nil && exit != nil {
 				select {
 				case exit <- err:
 				default: