avcodec/v4l2_context: Drop empty packet while draining

v4l2_m2m devices may send an empty packet/frame while draining
to indicate that all capture buffers have been flushed.

Currently, the empty packet/frame is not handled correctly:
When encoding, the empty packet is forwarded to the muxer, usually
creating warnings.
When decoding, a reference to the memory is created anyway. Since in
the past this memory contained a decoded frame, it results in an extra
frame being decoded.

This commit discards the empty packet/frame.

References:
linux/Documentation/media/uapi/v4l/dev-decoder.rst:

    "The last buffer may be empty (with :c:type:`v4l2_buffer` bytesused = 0)
     and in that case it must be ignored by the client, as it does not
     contain a decoded frame."

linux/Documentation/media/uapi/media/v4l/vidioc-encoder-cmd.rst:

    "...This buffer may be empty, indicated by the
     driver setting the ``bytesused`` field to 0."
Reviewed-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>

libavcodec/v4l2_context.c

@@ -405,6 +405,15 @@ dequeue:
             return NULL;
         }
 
+        if (ctx_to_m2mctx(ctx)->draining && !V4L2_TYPE_IS_OUTPUT(ctx->type)) {
+            int bytesused = V4L2_TYPE_IS_MULTIPLANAR(buf.type) ?
+                            buf.m.planes[0].bytesused : buf.bytesused;
+            if (bytesused == 0) {
+                ctx->done = 1;
+                return NULL;
+            }
+        }
+
         avbuf = &ctx->buffers[buf.index];
         avbuf->status = V4L2BUF_AVAILABLE;
         avbuf->buf = buf;