Send a null AVFrame to indicate end of stream to allow renderers to do render thread cleanup

app/streaming/video/ffmpeg-renderers/mmal.cpp

@@ -147,6 +147,11 @@ bool MmalRenderer::needsTestFrame()
 
 void MmalRenderer::renderFrame(AVFrame* frame)
 {
+    if (frame == nullptr) {
+        // End of stream - nothing to do for us
+        return;
+    }
+
     MMAL_BUFFER_HEADER_T* buffer = (MMAL_BUFFER_HEADER_T*)frame->data[3];
     MMAL_STATUS_T status;