v4l2: v4l2_camera_proxy: Don't allow streamon if no buffers have been requested

Make VIDIOC_STREAMON return -EINVAL if no buffers have been allocated
with reqbufs.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/v4l2/v4l2_camera_proxy.cpp

@@ -630,6 +630,9 @@ int V4L2CameraProxy::vidioc_streamon(V4L2CameraFile *file, int *arg)
 {
 	LOG(V4L2Compat, Debug) << "Servicing vidioc_streamon fd = " << file->efd();
 
+	if (bufferCount_ == 0)
+		return -EINVAL;
+
 	if (!validateBufferType(*arg))
 		return -EINVAL;