avdemux: Never return 0 from read function

Instead return AVERROR_EOF. The read function must never ever return 0 according
to the documentation, and in practice this leads to infinite loops.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/3369

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7390>

subprojects/gst-libav/ext/libav/gstavprotocol.c

@@ -325,7 +325,7 @@ gst_ffmpeg_pipe_read (void *priv_data, uint8_t * buf, int size)
   }
   GST_FFMPEG_PIPE_MUTEX_UNLOCK (ffpipe);
 
-  return size;
+  return size > 0 ? size : AVERROR_EOF;
 }
 
 int