server: fix crash that happens when a reader tries to setup a closed stream (https://github.com/aler9/rtsp-simple-server/issues/866)

serverstream.go

@@ -1,6 +1,7 @@
 package gortsplib
 
 import (
+	"fmt"
 	"sync"
 	"time"
 
@@ -120,6 +121,10 @@ func (st *ServerStream) readerAdd(
 	st.mutex.Lock()
 	defer st.mutex.Unlock()
 
+	if st.readers == nil {
+		return fmt.Errorf("stream is closed")
+	}
+
 	if st.s == nil {
 		st.s = ss.s