v4l2: v4l2_camera_proxy: Fix segfault on restarting streams

The V4L2 compatibility layer keeps track of the index of the next buffer
to be dequeued, to handle VIDIOC_DQBUF. This index is set to 0 on
startup and incremented (modulo #frames), and is otherwise never reset.
This means that if the last handled frame index is not #frames-1, and
the stream is restarted without restarting libcamera and the V4L2
compatilibity layer, the buffer index number won't match with the
libcamera buffer index number, causing a segfault. Fix this by resetting
the current buffer index to zero on VIDIOC_STREAMON.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/v4l2/v4l2_camera_proxy.cpp

@@ -465,6 +465,8 @@ int V4L2CameraProxy::vidioc_streamon(int *arg)
 	if (!validateBufferType(*arg))
 		return -EINVAL;
 
+	currentBuf_ = 0;
+
 	return vcam_->streamOn();
 }