backends/x11: Fix a crash in KWin::X11WindowedEglPrimaryLayer::present()

CompositorWayland will call the present() function even if the beginFrame()
fails, in which case, m_buffer can be null. Also, the present function
has no code to reset m_buffer after it has finished using it.

SENTRY: KWIN-6JD

src/backends/x11/windowed/x11_windowed_egl_backend.cpp

@@ -76,6 +76,10 @@ bool X11WindowedEglPrimaryLayer::doEndFrame(const QRegion &renderedRegion, const
 
 void X11WindowedEglPrimaryLayer::present()
 {
+    if (!m_buffer) {
+        return;
+    }
+
     xcb_pixmap_t pixmap = m_output->importBuffer(m_buffer->buffer());
     Q_ASSERT(pixmap != XCB_PIXMAP_NONE);
 
@@ -105,6 +109,7 @@ void X11WindowedEglPrimaryLayer::present()
 
     EGLNativeFence releaseFence{m_backend->eglDisplayObject()};
     m_swapchain->release(m_buffer, releaseFence.fileDescriptor().duplicate());
+    m_buffer = nullptr;
 }
 
 std::shared_ptr<GLTexture> X11WindowedEglPrimaryLayer::texture() const