VP9 denoiser bugfix in debugging code.

When OUTPUT_YUV_DENOISED is enabled the encoder outputs the uncompressed,
denoised video to a separate file. Moved the point at which the file is
written to in order to avoid an extra blank frame at the beginning of the video.

Change-Id: I805f6a912b18b3d9cae59b13c5b8108279439ce3