avcodec/videotoolboxenc: fix pixel buffer memory leak

In function vtenc_populate_extradata(), there is a manually created
pixel buffer that has not been released. So we should use CVPixelBufferRelease
to release this pixel buffer at the end, otherwise will cause a memory leak.

Signed-off-by: Rick Kern <kernrj@gmail.com>

libavcodec/videotoolboxenc.c

@@ -2554,6 +2554,7 @@ static int vtenc_populate_extradata(AVCodecContext   *avctx,
 
 
 pe_cleanup:
+    CVPixelBufferRelease(pix_buf);
     if(vtctx->session)
         CFRelease(vtctx->session);