directrenderer: prevent multiple potential video crashes

Avoid deleting the frame buffer while it's being
accessed by daemon or another DirectRenderer wrapper.

Change-Id: I8c905d0fe274c9b546a7b3438b8a0cbddf67f456

src/directrenderer.cpp

-/****************************************************************************
- *    Copyright (C) 2012-2021 Savoir-faire Linux Inc.                          *
+/****************************************************************************
+ *   Copyright (C) 2012-2021 Savoir-faire Linux Inc.                        *
  *   Author : Alexandre Lision <alexandre.lision@savoirfairelinux.com>      *
  *   Author : Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>      *
  *                                                                          *
@@ -89,6 +89,7 @@ Video::DirectRenderer::DirectRenderer(const QString& id, const QSize& res, bool
 /// Destructor
 Video::DirectRenderer::~DirectRenderer()
 {
+    QMutexLocker lk(mutex());
     stopRendering();
 
     d_ptr.reset();